[Diagnostics] Fix inconsistent shift-overflow warnings in C++20

This fixes https://github.com/llvm/llvm-project/issues/52873.
Don't warn in C++2A mode (and newer), as signed left shifts
always wrap and never overflow. Ref. -
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1236r1.html.