Generalize the 80-bit long double in <complex> for Clang (#1728)

stl/inc/complex

@@ -540,7 +540,7 @@ public:
     }
 
     static bool _Isinf(_Ty _Left) { // test for infinity
-#if defined(__INTEL_COMPILER) && defined(__LONG_DOUBLE_SIZE__) && __LONG_DOUBLE_SIZE__ == 80
+#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18
         return _CSTD _LDtest(&_Left) == _INFCODE;
 #else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv
         const auto _Uint = _Bit_cast<uint64_t>(_Left);
@@ -549,7 +549,7 @@ public:
     }
 
     static _CONSTEXPR20 bool _Isnan(_Ty _Left) {
-#if defined(__INTEL_COMPILER) && defined(__LONG_DOUBLE_SIZE__) && __LONG_DOUBLE_SIZE__ == 80
+#if defined(__LDBL_DIG__) && __LDBL_DIG__ == 18
         return _CSTD _LDtest(&_Left) == _NANCODE;
 #else // ^^^ 80-bit long double (not supported by MSVC in general, see GH-1316) / 64-bit long double vvv
         const auto _Uint = _Bit_cast<uint64_t>(_Left);