When instantiating C++ types who contain ARC types in the past either
their default constructors are skipped or they are synthesized with
Unmanged<> wrappers. This has made it more cumbersome to instantiate
these from Swift and often requires a static C++ build method to
manually shim the synthesized constructor. The following change drops
the Unmanaged<> so that for the time being these types can be
instantiated directly even if types like Swift -> NSString are not
bridged completely.
(cherry picked from commit accdace5f0438860a0d6760d9598402a9998a324)
(cherry picked from commit 567763c9ec76e14b0d6af5707e50b1674b2c2cbf)
