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

[Justin Bogner via cfe-commits]

bogner wrote:

This is NFC in practice so it might be okay that I don't have any tests, but if folks have ideas on how to test this I'd like that better.

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