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

[Stephen Tozer via cfe-commits]

SLTozer wrote:

> looks good - a few bits should be committed separately from this change, so please do those first and then commit this change

Separate review opened up at: https://github.com/llvm/llvm-project/pull/68967

For simplicity's sake I'll keep this patch as-is, and rebase it when the other patch lands.

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