[clang] [llvm] [PAC][Driver] Implement `-mbranch-protection=pauthabi` option (PR #97237)

[Anton Korobeynikov via cfe-commits]

asl wrote:

> Is there any thought on how we want to manage signing schemas going forward? For example I can imagine looking an environment from the triple to select a signing schema for a particular platform. I could also see a potential for a separate command line option to choose from documented named signing schemas.

I think we discussed this a bit and the conclusion was that it would be up to the platform to chose the appropriate signing scheme (and do ABI versioning if desired / necessary). However, currently there is no way to ask platform for this and it looks like a chicken-and-egg problem. So we may want to come with some "default" values that are more or less "good enough". These in the future might be fully overriden by a platform or we may chose alternative approach. For now it is just a way to combine different `-fpauth-*` options as it seems to me.

https://github.com/llvm/llvm-project/pull/97237