[clang] [Clang] Add [[clang::no_specializations]] (PR #101469)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I went for only adding `[[clang::no_specialization]]` to function templates, since the other cases seem even more unlikely to me and seem to have a higher implementation complexity. Is that OK?

Your test coverage also handles class templates and variable templates in addition to function templates, which I think covers the most common cases and is fine.

https://github.com/llvm/llvm-project/pull/101469