configure: when performing a compiler feature test for a -Wno-foo flag, attempt

to use -Wfoo instead of -Wno-foo. This works around a bug in some versions of
gcc, where it will silently accept an unknown -Wno-foo option, but will
generate an error for a compile which uses -Wno-foo if that compile also
triggers any warnings.

llvm-svn: 174770