Further refine -Wunreachable-code groups so that -Wno-unreachable-code-break doesn't turn off all unreachable code warnings.

Also relax unreachable 'break' and 'return' to not check for being
preceded by a call to 'noreturn'.  That turns out to not be so
interesting in practice.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@204000 91177308-0d34-0410-b5e6-96231b3b80d8