swift234

Change swift_retain/swift_retain_n to return no value. this is part of a series of commits
to remove reference forwarding for some of the ARC entry points. rdar://22724641. After this
commit, swift_retain will be the same as swift_retain_noresult, returning no reference.
LLVMARCContract pass is also modified NOT to rewrite swift_retain_noresult to the
old swift_retain which forwards the reference.

Swift SVN r32055