[clang] Fix evaluation of the unsigned enumeration values, #108766 (PR #108769)

[Dmitry Fursov via cfe-commits]

fursov wrote:

@DeinAlptraum, thank you for the review. The findings are fixed now.
Could you say if the fix to be backported to other relelases, like 18.x or 19rc? If yes, how this is done?

https://github.com/llvm/llvm-project/pull/108769