Previously, we were just running ObjCARCContract when codegening. This is out of
character with the rest of the LLVM passes in Swift, namely that after these
have run, an IR pass no longer contains any compiler intrinsics. This can be
seen by SwiftARCContract running in the -O pipeline.
This commit harmonizes the behavior here.
For testing purposes, I added a flag that disables the running of
ObjCARCContract for testing purposes.
rdar://34824507
