Further refine -Wunreachable-code groups so that -Wno-unreachable-code-break...

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.

llvm-svn: 204000