[Clang] __builtin_memset should be constexpr

`__builtin_memset` currently isn't `constexpr`, but could be AFAICT. There doesn't seem to be a good reason to not allow at least single byte integral types.

Start by investigating Clang's handling of __builtin_memset and how constexpr evaluation treats it. The issue proposes allowing constexpr use for at least single-byte integral types; done means that supported use is accepted without breaking existing compiler behavior.

Written by the indexing model from the issue text.