[clang] Suppress errors from well-formed-testing type traits in SFINAE contexts (PR #135390)

[Aaron Puchert via cfe-commits]

aaronpuchert wrote:

> (It should be noted that the standard doesn't always base this on the immediate context being well-formed: for `std::common_type` it's based on whether some expression "denotes a valid type." But I assume that's an editorial issue and means the same thing.)

Filed cplusplus/draft#7827 to clarify this.

https://github.com/llvm/llvm-project/pull/135390