[clang] [clang-tools-extra] [clang] Improve nested name specifier AST representation (PR #147835)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> Seems like `getOriginalDecl()` always returns the full definition for enumerations (and not an opaque declaration for enums with fixed underlying type), at least in the C language mode. Is it guaranteed?

I don't see that happening. Example: https://compiler-explorer.com/z/z9cMxPWva

https://github.com/llvm/llvm-project/pull/147835