Changed CGObjCMac.cpp to add the marker externally_initialized to SELECTOR_REFERENCES...

Changed CGObjCMac.cpp to add the marker externally_initialized to SELECTOR_REFERENCES in both the fragile and non-fragile API.

This is to ensure that GlobalOpt in LLVM does not attempt to look through a
selector reference to a method var name at compile time.

I also added a test/updated old tests that need to recognize the new keyword.

rdar://12580965.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174461 91177308-0d34-0410-b5e6-96231b3b80d8