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

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Sirraide 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.

Yeah, I also remember finding that strange, but after looking into it I figured it made sense, but I don’t remmeber why exactly...

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