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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156937 91177308-0d34-0410-b5e6-96231b3b80d8