swift234

IRGen: Fix partial apply of functions with parametric return type

In partial apply forwarding thunks,  we need to cast dependent result types of
polymorphic function calls to the specialized type.

For example for:

 %0 = function_ref @closure :
         $@thin <C where C : Base> (@owned Base) -> @owned C
 %1 = partial_apply %0<Sub>() :
         $@thin <C where C : Base> (@owned Base) -> @owned C

We need to cast the result type:

  define %C3Sub* @_TPA_closure(%C4Base*) {
  entry:
    %1 = call %swift.type* @_TMaC3Sub()
    %2 = tail call %C4Base* @closure(%C4Base* %0, %swift.type* %1)
-->  %3 = bitcast %C4Base* %2 to %C3Sub*
    ret %C3Sub* %3
  }

rdar://19930705

Swift SVN r25521