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

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> I think we should also adjust the description of FeatureHCX as it no longer enables any register. I was thinking about just saying that this flag doesn't do anything and is only present for legacy reasons. What do you think about that ?

I can do, but it has large ripple effects:
```
% rg FEAT_HCX ./clang/test/Driver/print-enabled-extensions/* | wc -l
33
```
Would require changes to 33 files.

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