Warn about -Wno-foo where foo is an unknown warning option. This is helpful

for subtle misspellings such as -Wno-unused-command-line-arguments instead of
-Wno-unused-command-line-argument.

Also fix the diagnostic messages to properly handle -Wno- options.  Previously,
the positive version was always emitted (i.e., -Wfoo was emitted for -Wno-foo).
rdar://11461500

llvm-svn: 156937