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

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Lukacma wrote:

To me it seems like, the simpler route is to remove feature guarding from system register, as is our current direction. With that change we can completely get rid of this feature, which to me is better choice than adding it to list of supported extensions.

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