[clang] [clang-tools-extra] [Attributes][HLSL] Teach EnumArgument to refer to an external enum (PR #70835)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Its a bit of a shame that the `VariadicEnumArgument` isn't being included here/updated here, I typically appreciate the variadic versions being kept in sync.  But since we don't have a use for it at the moment, I think a 'fixme' (or a future patch!) would be acceptable.

https://github.com/llvm/llvm-project/pull/70835