[clang] makes `__is_trivially_equality_comparable` available as a struct

[cjdb]

Summary

Since this was originally a name in library, it needs an escape hatch
for versions of Clang that are slightly out-of-sync with libc++.

[cjdb]

uploads the right test

clang/lib/Parse/ParseDeclCXX.cpp

#include "clang/Basic/TransformTypeTraits.def"
           tok::kw___is_rvalue_reference,
           tok::kw___is_same,
           tok::kw___is_scalar,
           tok::kw___is_scoped_enum,
           tok::kw___is_sealed,
           tok::kw___is_signed,
           tok::kw___is_standard_layout,
           tok::kw___is_trivial,
+          tok::kw___is_trivially_equality_comparable,
           tok::kw___is_trivially_assignable,
           tok::kw___is_trivially_constructible,
           tok::kw___is_trivially_copyable,
           tok::kw___is_unbounded_array,
           tok::kw___is_union,
           tok::kw___is_unsigned,
           tok::kw___is_void,
           tok::kw___is_volatile))

clang/test/SemaCXX/libcxx_is_trivially_equality_comparable_hack.cpp

+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fsyntax-only -verify -std=gnu++20 -fms-extensions -Wno-microsoft %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fsyntax-only -verify -std=gnu++2b -fms-extensions -Wno-microsoft %s
+
+template <class T, class U>
+struct Same {
+  static constexpr auto value = __is_same(T, U);
+};
+
+template <class T>
+struct __is_trivially_equality_comparable { // expected-warning{{keyword '__is_trivially_equality_comparable' will be made available as an identifier for the remainder of the translation unit}}
+  using type = T;
+};
+
+using A = Same<__is_trivially_equality_comparable<int>::type, __is_trivially_equality_comparable<int>::type>;