swift234

Enable the autorelease reclaim optimization for real
by not doing various dumb things that break it.

Specifically, directly return the result of
objc_autoreleaseReturnValue (so that we can actually
tail call it) and substitute the result of
objc_retainAutoreleasedReturnValue for the call result
(so that we don't have instructions saving the call
result interfering with the pattern being sniffed).

I found both of those bugs by educated inspection, then
spent a while debugging why it wasn't making my test case
successfully reclaim.  It turns out that the first
autorelease reclaim attempt will always fail on x86-64
because the sniffing code finds the dyld stub; the
optimization only actually works after
objc_retainAutoreleaseReturnValue has been called at
least once in that shared object.  Fun times.

rdar://16565958

Swift SVN r16351