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

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

Thanks for the reviews.

> Looks good; I understand we have to support driver flags forever because they get baked into peoples build systems. Would we be able to get away with a cc1 flag instead?

> I suppose this strays into the topic of "how are we going to deploy this", which as everything is behind a compile time flag, we probably don't need to immediately answer. IMO we don't have to support driver flags forever that were in turn behind their own experimental compile-time flag.

@jmorse Does the in-line conversation change your stance on this? If not, it would be easier to make this cc1 after I've uploaded bitcode handling patches, if it's alright to have it in this state temporarily to keep things moving - as all the front end patches are blocked on this one.

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