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

[Aaron Ballman via cfe-commits]

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

LGTM with unrelated changes pulled; I'm fine landing this PR after landing the fix for the function pointer classification changes if necessary.

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