Speculatively implement LWG-3867 (#3367)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/syncstream

@@ -346,7 +346,7 @@ public:
 
     ~basic_osyncstream() = default;
 
-    basic_osyncstream& operator=(basic_osyncstream&&) noexcept = default;
+    basic_osyncstream& operator=(basic_osyncstream&&) = default; // per LWG-3867
 
     void emit() {
         ios_base::iostate _State = ios_base::goodbit;

tests/std/tests/GH_001858_iostream_exception/test.cpp

@@ -389,8 +389,6 @@ static_assert(is_nothrow_swappable_v<wstringstream>);
 static_assert(is_nothrow_swappable_v<syncbuf>);
 static_assert(is_nothrow_swappable_v<wsyncbuf>);
 
-// TRANSITION, GH-3355: "<syncstream>: is_nothrow_move_assignable_v<osyncstream> is squirrelly"
-
 static_assert(has_nothrow_rdbuf<osyncstream>); // mandatory
 static_assert(has_nothrow_rdbuf<wosyncstream>); // mandatory
 #endif // _HAS_CXX20