SILGen: Implement function conversions involving opened existentials

Suppose we have a protocol requirement returning Self:

protocol Clonable {
  func clone() -> Self
}

If we have a value 'x' of existential type 'Clonable',
the partial application 'x.clone' has formal type
'() -> Clonable'. However the actual type of the
method substitutes in an "opened" existential type
for 'Self'.

In order to implement the partial application we must
wrap the method in a thunk which 'erases' the opened
existential, so the thunk has type

<T : Clonable> (() -> T) -> () -> Clonable

The thunk is called with a substitution replacing 'T'
with the opened existential type.

Fixes <rdar://problem/21391055>.

lib/SILGen/SILGenFunction.h

@@ -1535,7 +1535,9 @@ public:
   CanSILFunctionType buildThunkType(ManagedValue fn,
                                     CanSILFunctionType expectedType,
                                     CanSILFunctionType &substFnType,
-                                    SmallVectorImpl<Substitution> &subs);
+                                    GenericEnvironment *&genericEnv,
+                                    SubstitutionMap &contextSubMap,
+                                    SubstitutionMap &interfaceSubMap);
 
   //===--------------------------------------------------------------------===//
   // Declarations