[clang] [clang] Use new interpreter in EvaluateAsConstantExpr if requested (PR #70763)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't have a good idea how a release note could be phrased here, but if you could try one, it would be appreciated and I think it is worth doing.

Side note: it would be really cool if at the end of the release cycle, you did a section in the release-notes with a 'status' of the new interpreter.  Basically a 'things that don't work', or a 'percent of tests we have that work', etc?  

https://github.com/llvm/llvm-project/pull/70763