In Objective-C, ignore attempts to redefine the ARC/GC qualifier macros.

This works around existing system headers which unconditionally
redefine these macros.

This is reasonably safe to do because we used to warn about it anyway
(outside of system headers).  Continue to warn if the redefinition
would have changed the expansion.  Still permit redefinition if the
macro is explicitly #undef'ed first.

rdar://23788307

llvm-svn: 255311