[clang] [clang] Validate ABI tag attributes (PR #84272)

[Victor Chernyakin via cfe-commits]

LocalSpook wrote:

> I kind of think "no enforcement for invalid input" is reasonable here. That would be consistent with things like the alias attribute where we accept a string that has to conform to ABI mangling rules (which could be a matter of poor QoI rather than precedent).

If I'm not mistaken, `alias` does *not* accept invalid input, because the compiler ensures the given string matches the mangled name of another entity in the program (whose name was checked and mangled by Clang and is therefore correct).

https://github.com/llvm/llvm-project/pull/84272