[clang] [KeyInstr][Clang] Add Clang option -g[no-]key-instructions (PR #134627)

[Jeremy Morse via cfe-commits]

https://github.com/jmorse approved this pull request.

LGTM then, in practice we always get bitten by commandlines + LTO sooner or later, so I'm confident we'll end up doing-the-right-thing anyway after breaking our internal CI a few times.

https://github.com/llvm/llvm-project/pull/134627