Fix -fobjc-arc doesn't work with -save-temps

The underlying problem is that there is currently no way to run
ObjCARCContract from llvm bitcode which is required by ObjC ARC.
This fix the problem by always enable ObjCARCContract pass if
optimization is enabled. The ObjCARC Contract pass has almost no
overhead on code that is not using ARC.

llvm-svn: 236372