[clang] [clang] Fix implicit integer conversion for opaque enum declarations in class templates (PR #121039)

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thebrandre wrote:

I added regression tests for the scoped enumeration types to make sure that these are not affected.
And I also tried to improve the commit message and added comments with references to the relevant sections in the C++ standard.
But I still have to think about your refactoring/completeness suggestions.

https://github.com/llvm/llvm-project/pull/121039