[clang] [Clang][ExprConstant] fix constant expression did not evaluate to integer (PR #97146)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

To the extent that EvaluateAsRValue handles LValues, FastEvaluateAsRValue should also handle them, sure.  I'm suspicious of the fact that EvaluateAsRValue is doing this conversion in the first place, but I guess this pull request isn't the place to address that.

Given that, this patch LGTM

https://github.com/llvm/llvm-project/pull/97146