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

[Stephen Tozer via cfe-commits]

SLTozer wrote:

Making this attribute mutually exclusive with `optdebug` and `optsize` is fine with me - as a question for the esteemed reviewers of this patch, would it be preferred to add this exclusivity and associated verifier checks in a separate patch, or as part of this patch?

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