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

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

This now lacks my concerns, as this is only applying to the singular expression.  But parsing isn't my expertise, so @cor3ntin or @Endilll should probably think about this for a minute or two.

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