[clang] [LLVM] Add new attribute `optdebug` to optimize for debugging (PR #66632)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

> Are there interactions between `optdebug` and other attributes that should be enforced by the verifier? Like, can't be mixed with `optnone`?

+1, this is basically "I want `-O1`" vs "I want `-O0`" so they must conflict

https://github.com/llvm/llvm-project/pull/66632