Bits of cleanup from clang 10 investigation (#475)

I built clang from the `release/10.x` branch to investigate support for the portions of the STL that require concepts. This went swimmingly, until I was blocked by LLVM-44627 "Reversed candidate operator is not found by argument dependent lookup". These are a few fixes and workarounds I discovered in the process of getting some tests to pass before being blocked completely.

Detailed changes:

`<compare>`:
* Workaround LLVM-41991 "c++2a: attributes on defaulted friend functions incorrectly rejected" by using `_NODISCARD` on such functions only for non-clang.

`<concepts>`:
* Fix typo.
* Workaround LLVM-44689 "[concepts] ICE when *this appears in trailing requires-clause" in `std::ranges::swap`.

`<xhash>`:
* Silence clang warning about template parameter shadowing by renaming.

`<xutility>`:
* Clang thinks my `unreachable_sentinel_t` hack is ill-formed.

stl/inc/compare

@@ -50,7 +50,10 @@ public:
     }
 
 #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201902L
-    _NODISCARD friend constexpr bool operator==(const partial_ordering&, const partial_ordering&) noexcept = default;
+#ifndef __clang__ // TRANSITION, LLVM-41991
+    _NODISCARD
+#endif // TRANSITION, LLVM-41991
+    friend constexpr bool operator==(const partial_ordering&, const partial_ordering&) noexcept = default;
 #else // ^^^ supports <=> and P1185 / supports neither vvv
     _NODISCARD friend constexpr bool operator!=(const partial_ordering _Val, _Literal_zero) noexcept {
         return !_Val._Is_ordered || _Val._Value != 0;
@@ -138,7 +141,10 @@ public:
     }
 
 #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201902L
-    _NODISCARD friend constexpr bool operator==(const weak_ordering&, const weak_ordering&) noexcept = default;
+#ifndef __clang__ // TRANSITION, LLVM-41991
+    _NODISCARD
+#endif // TRANSITION, LLVM-41991
+    friend constexpr bool operator==(const weak_ordering&, const weak_ordering&) noexcept = default;
 #else // ^^^ supports <=> and P1185 / supports neither vvv
     _NODISCARD friend constexpr bool operator!=(const weak_ordering _Val, _Literal_zero) noexcept {
         return _Val._Value != 0;
@@ -229,7 +235,10 @@ public:
     }
 
 #if defined(__cpp_impl_three_way_comparison) && __cpp_impl_three_way_comparison >= 201902L
-    _NODISCARD friend constexpr bool operator==(const strong_ordering&, const strong_ordering&) noexcept = default;
+#ifndef __clang__ // TRANSITION, LLVM-41991
+    _NODISCARD
+#endif // TRANSITION, LLVM-41991
+    friend constexpr bool operator==(const strong_ordering&, const strong_ordering&) noexcept = default;
 #else // ^^^ supports <=> and P1185 / supports neither vvv
     _NODISCARD friend constexpr bool operator!=(const strong_ordering _Val, _Literal_zero) noexcept {
         return _Val._Value != 0;

stl/inc/concepts

@@ -32,7 +32,7 @@ _STD_BEGIN
 template <class _Ty1, class _Ty2>
 concept _Same_impl = // Must be a distinct concept to provide symmetric subsumption for same_as
 #ifdef __clang__
-    __is_same(_Ty1 _Ty2);
+    __is_same(_Ty1, _Ty2);
 #else // ^^^ use intrinsic / no intrinsic vvv
     is_same_v<_Ty1, _Ty2>;
 #endif // __clang__
@@ -160,7 +160,7 @@ namespace ranges {
             template <class _Ty1, class _Ty2, size_t _Size>
             constexpr void operator()(_Ty1 (&__t)[_Size], _Ty2 (&__u)[_Size]) const
                 noexcept(noexcept((*this)(__t[0], __u[0])))
-                requires requires { (*this)(__t[0], __u[0]); } {
+                requires requires(const _Cpo& __f) { __f(__t[0], __u[0]); } { // TRANSITION, LLVM-44689
                 for (size_t __i = 0; __i < _Size; ++__i) {
                     (*this)(__t[__i], __u[__i]);
                 }

stl/inc/xhash

@@ -376,15 +376,15 @@ public:
     static constexpr bool _Multi            = _Traits::_Multi;
 
 #if !_HAS_IF_CONSTEXPR
-    template <class _Traits>
-    friend bool _Hash_equal_elements(const _Hash<_Traits>& _Left, const _Hash<_Traits>& _Right, false_type);
+    template <class _TraitsT>
+    friend bool _Hash_equal_elements(const _Hash<_TraitsT>& _Left, const _Hash<_TraitsT>& _Right, false_type);
 
-    template <class _Traits>
-    friend bool _Hash_equal_elements(const _Hash<_Traits>& _Left, const _Hash<_Traits>& _Right, true_type);
+    template <class _TraitsT>
+    friend bool _Hash_equal_elements(const _Hash<_TraitsT>& _Left, const _Hash<_TraitsT>& _Right, true_type);
 #endif // !_HAS_IF_CONSTEXPR
 
-    template <class _Traits>
-    friend bool _Hash_equal(const _Hash<_Traits>& _Left, const _Hash<_Traits>& _Right);
+    template <class _TraitsT>
+    friend bool _Hash_equal(const _Hash<_TraitsT>& _Left, const _Hash<_TraitsT>& _Right);
 
 protected:
     _Hash(const _Key_compare& _Parg, const allocator_type& _Al)

stl/inc/xutility

@@ -3490,21 +3490,21 @@ struct unreachable_sentinel_t;
 namespace _Unreachable_sentinel_detail {
     struct _Base {
         template <weakly_incrementable _Winc>
-        _NODISCARD friend constexpr bool operator==(unreachable_sentinel_t, const _Winc&) noexcept {
+        _NODISCARD friend constexpr bool operator==(const unreachable_sentinel_t&, const _Winc&) noexcept {
             return false;
         }
 #if !defined(__cpp_impl_three_way_comparison) || __cpp_impl_three_way_comparison < 201902L
         template <weakly_incrementable _Winc>
-        _NODISCARD friend constexpr bool operator==(const _Winc&, unreachable_sentinel_t) noexcept {
+        _NODISCARD friend constexpr bool operator==(const _Winc&, const unreachable_sentinel_t&) noexcept {
             return false;
         }
 
         template <weakly_incrementable _Winc>
-        _NODISCARD friend constexpr bool operator!=(unreachable_sentinel_t, const _Winc&) noexcept {
+        _NODISCARD friend constexpr bool operator!=(const unreachable_sentinel_t&, const _Winc&) noexcept {
             return true;
         }
         template <weakly_incrementable _Winc>
-        _NODISCARD friend constexpr bool operator!=(const _Winc&, unreachable_sentinel_t) noexcept {
+        _NODISCARD friend constexpr bool operator!=(const _Winc&, const unreachable_sentinel_t&) noexcept {
             return true;
         }
 #endif // !defined(__cpp_impl_three_way_comparison) || __cpp_impl_three_way_comparison < 201902L