Change the diagnostic group for unsupported gcc optimizations added r212805

Use -Winvalid-command-line-argument here to align with existing gcc opt
diagnostics. Meanwhile -Wunused-command-line-argument is for flags that we
support but were, say, fed into the wrong invocation.

Also tweak wording to make sense with -Werror.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212964 91177308-0d34-0410-b5e6-96231b3b80d8