Marking the objc_autoreleaseReturnValue and objc_retainAutoreleaseReturnValue

call sites as tail calls unconditionally.  While it's theoretically true that
this is just an optimization, it's an optimization that we very much want to
happen even at -O0, or else ARC applications become substantially harder to
debug.  See r169796 for the llvm/fast-isel side of things.
rdar://12553082

llvm-svn: 169996