[clang] [llvm] [AArch64][clang][llvm] Allow +hcx to be specified as optional for Armv8.6 (PR #208019)

[via cfe-commits]

Lukacma wrote:

> Maybe in future we could have LegacyExtension in AArch64Features.td so that anything using this would still be accepted, but not listed with --print-supported-features, etc. We could do this at leisure in a separate PR

Yeah we have more registers guarrded by features and probably more features will need to be retired once we remove the gating so having some sort of system for that would be nice. 

https://github.com/llvm/llvm-project/pull/208019