[clang] [Clang] use constant evaluation context for constexpr if conditions (PR #123667)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik requested changes to this pull request.

Please add a more detailed summary to this PR. I have asked this on several PRs and I believe the confusion reflected in the comments is very much attributed to not explaining why you believe your fix works. It may be incorrect but a solid summary will allow the reviewers to quickly understand the assumptions you are making in your fix and should reduce back and forth and allow for more targeted feedback quicker.

https://github.com/llvm/llvm-project/pull/123667