[clang] [Clang] Properly parse enumeration declarations with fixed underlying type in generic associations (PR #210353)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

I have no idea why you put `Fixed #207357` in the PR title, which failed to make use of [GitHub's close/fix/resolve syntax](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword). I moved it to the PR description and reworded the title. Please double check the changes.

https://github.com/llvm/llvm-project/pull/210353