Release/retain functions for C++ foreign reference types might return a reference count as an integer value.
Swift previously emitted an error for such functions, saying that the retain/release functions need to return void or a reference to the value.
rdar://157853183
(cherry picked from commit 3887ed0637)
Many existing C APIs for retaining references, including Apple's own, return
the reference. Support this pattern, along with the existing void return
signature, with when importing reference types from C++.
