[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:

Just 1 more comment (the name of `External` everywhere), else LGTM.

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