[-Wunreachable-code] Prune out unreachable warnings where a 'break' is preceded by...

[-Wunreachable-code] Prune out unreachable warnings where a 'break' is preceded by a call to a 'noreturn' function.

For example:

	unreachable();
    break;

This code is idiomatic and defensive.  The fact that 'break' is
unreachable here is not interesting.  This occurs frequently
in LLVM/Clang itself.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202328 91177308-0d34-0410-b5e6-96231b3b80d8