[CodeGen] Mark calls to objc_autorelease as tail

This enables a method sending an autorelease message to an object and
returning the object in MRR to avoid adding the object to an autorelease
pool if a call to objc_retainAutoreleasedReturnValue in the caller
function accepts the hand off of the retain count.

rdar://problem/50678052

Differential Revision: https://reviews.llvm.org/D91111