Changed "an macro" to "a macro" in diagnostic note.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186988 91177308-0d34-0410-b5e6-96231b3b80d8
Remove HelpText for -fno-diagnostics-show-note-include-stack

The help text was wrong, and we already provide help text on the
option that actually changes anything, i.e. -fdiagnostics-show-note-include-stack.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186989 91177308-0d34-0410-b5e6-96231b3b80d8