[clang] [Sema] Add code completion for if constexpr (PR #124315)

[Sirui Mu via cfe-commits]

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

Thanks for working on this! Please add a test for your patch. You can take tests under `clang/test/CodeCompletion` for reference.

https://github.com/llvm/llvm-project/pull/124315