[libc++abi] Add -Wno-unreachable-code when building test for throwing incomplete types

Slightly older Clangs seem to think they are more clever than they really
are, and they think the code can never be executed. The code can actually
be executed in case the exception runtime is mis-implemented, which is
exactly what this test is testing. This commit just disables the spurious
warning.

GitOrigin-RevId: 78769923fe03b6d8dfcd3dad69b6f3eff5a2f813
1 file changed