[clang] [Clang] [Parser] Support [[omp::assume]] (PR #84582)

[via cfe-commits]

Sirraide wrote:

@alexey-bataev I have a few more questions about `[[omp::assume]]`:
- Is this supported in C? Because the `[[]]` spelling is only officially available in C23 and later (we *do* support it in earlier language modes, but users will get a warning if `-pedantic` is passed)
- Is there any place where I can find some more documentation about this attribute? I tried to search for it online, but I couldn’t find anything about it.

https://github.com/llvm/llvm-project/pull/84582