[clang] [clang][AST][NFC] Add default value to `Expr::isConstantInitializer()` parameter (PR #190313)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

I still want to eventually get rid of isConstantInitializer, I think, in favor of some variation of Evaluate()... but this is fine for now.

https://github.com/llvm/llvm-project/pull/190313