Teach format string checking about compile-time CFString constants.

Within the guts of CheckFormatHandler, the IsObjCLiteral flag was being used in
two ways: to see if null bytes were allowed, and to see if the '%@' specifier
is allowed.* The former usage has been changed to an explicit test and the
latter pushed down to CheckPrintfHandler and renamed ObjCContext, since it
applies to CFStrings as well.

* This also changes how wide chars are interpreted; in OS X Foundation, the
wide character type is 'unichar', a typedef for short, rather than wchar_t.

llvm-svn: 157968