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

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

I think typically the verifier changes are done in the same change since they match the LangRef change, but it doesn't matter too much as long as it gets sdone.

The LangRef change should also mention the attributes this is incompatible with (like `optnone`'s description)

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