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

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> What do you think about that ?

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.

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