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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

#100452 only fixed the "ignoring the in-class initializer of the static data member" issue.  I don't think we did anything to allow downgrading overflow errors to warnings.

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