[clang][Sema] Avoid duplicate diagnostics for unreachable fallthrough attribute

This patch checks whether -Wunreachable-code-fallthrough is enabled
when clang encounters unreachable fallthrough attributes and, if so,
suppresses code will never be executed warning to avoid duplicate
warnings.

Fixes https://github.com/llvm/llvm-project/issues/60416
Differential Revision: https://reviews.llvm.org/D145842