[Clang] Allow __is_nothrow_convertible to be used as an identifier (llvm#80476)

`__is_nothrow_convertible` has been used by libstdc++ previously as an
identifier.

Author: philnik777

clang/lib/Parse/ParseDeclCXX.cpp

@@ -1725,6 +1725,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
           tok::kw___is_member_pointer,
           tok::kw___is_nothrow_assignable,
           tok::kw___is_nothrow_constructible,
+          tok::kw___is_nothrow_convertible,
           tok::kw___is_nothrow_destructible,
           tok::kw___is_nullptr,
           tok::kw___is_object,

clang/test/SemaCXX/libstdcxx_is_nothrow_convertible_hack.cpp

@@ -0,0 +1,14 @@
+// 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++23 -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_nothrow_convertible { // expected-warning{{keyword '__is_nothrow_convertible' will be made available as an identifier for the remainder of the translation unit}}
+  using type = T;
+};
+
+using A = Same<__is_nothrow_convertible<int>::type, __is_nothrow_convertible<int>::type>;