[clang] feat: included support for is_constant builtin (PR #166832)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

Hi @siddu0660. Thanks for working on this! I appreciate that you were proactive in getting started on this. Unfortunately, that probably resulted in a bit of duplicated effort as @adams381 is also working on this, but I can see that you had probably started before I added the comment in https://github.com/llvm/llvm-project/issues/163889 explaining that.

I'll review the PR in a minute, but my first feedback is that the title should begin with `[CIR]` for CIR-related work. This is obviously a minor point, but as this is your first contribution I'm going to point out many small things like this to introduce you to our usual practices.

https://github.com/llvm/llvm-project/pull/166832