[clang] [ExprConstant] Handle shift overflow the same way as other kinds of overflow (PR #99579)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

There's some existing coverage of the affected diagnostics, which shows we continue to produce those diagnostics... but I added a few more tests for in-class static member variables.

https://github.com/llvm/llvm-project/pull/99579