[llvm-config] Fix cflags test looking for "warning"

This will fail if you configure with e.g. -Wno-unknown-warning-option.
Change it to check for 'warning:' just like we did for 'error:' in
r289484.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304239 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed