SMDiagnostic: don't emit ranges if there are /any/ multibyte characters.

Right now, only OS X has a way to determine the column width of a string
(PR14910). Until we have a good way to deal with this, we just won't
print carets, source ranges, or fixits for SMDiagnostic if the source line
has multibyte characters in it.

llvm-svn: 172164