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

[André Brand via cfe-commits]

thebrandre wrote:

@cor3ntin Thanks! I just added some details to the GitHub description. I hope it's not too verbose.
And I rebased the branch to resolve the conflict in the release notes.

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