[clang] [clang] Emit bad shift warnings (PR #70307)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I think the issue is the usage of "Info.EvalStatus.Diag" to determine whether we consider the expression an ICE; that's going to lead to unpredictable results, and it's not how we handle things anywhere else.  Not sure what the goal of that change is.

Separately, Sema::AddInitializerToDecl should probably print diagnostics when it checks for an ICE, instead of just emitting the generic diag::err_in_class_initializer_non_constant/diag::ext_in_class_initializer_non_constant.  Any maybe we should consider enabling ext_in_class_initializer_non_constant by default.

https://github.com/llvm/llvm-project/pull/70307