[clang] [clang] Prevent duplicated instantiation of enumerators of unscoped member enumerations (PR #124407)

[André Brand via cfe-commits]

thebrandre wrote:

@cor3ntin I came across this while refactoring, which is still in progress because I'm coming across odd things like this that make me a bit uncertain. Sometimes it's hard to tell if its a bug or a feature.

https://github.com/llvm/llvm-project/pull/124407