clang: Support -Wswitch-unreachable

gcc 16 supports -Wswitch-unreachable
https://godbolt.org/z/b6KeT85P5

:6:5: warning: statement will never be executed [-Wswitch-unreachable]

Clang trunk doesn't show this warning under -Wall or -Wextra.

Gcc correctly doesn't show warning for nested loop/switch case:
https://godbolt.org/z/8Y8bjdWG8

Start by reproducing the GCC 16 behavior from the first Godbolt example and compare it with Clang trunk under -Wall and -Wextra. Use the nested loop/switch example to verify the warning is not emitted there; done means Clang supports -Wswitch-unreachable with the same distinction.

Written by the indexing model from the issue text.