[clang] Diagnose invalid specializations of type traits

[ldionne]

The following code is ill-formed, but we don't diagnose it:

#include <type_traits>

template <>
struct std::common_type<int, int> { using type = long; }; // should warn

struct Foo { };
template <>
struct std::common_type<Foo> { using type = int; }; // should warn

template <>
struct is_integral<Foo> { }; // should warn

// etc...

It would be nice if Clang caught invalid specializations of std::common_type, and in fact perhaps of the other type trait classes as well.

Extracted from #99473 (comment).

[MitalAshok]

Related: #101469 (libc++ can use this for the is_integral case to get the warning)