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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm concerned that we're calling EvaluateKnownConstInt on an lvalue; that might indicate there's something wrong with the AST.  Usually there should be an lvalue-to-rvalue cast somewhere.

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