Make IUO unwraps in thunks use the IUO message

@jckarter pointed out that force unwraps emitted in thunks are always for IUOs and should use the implicit unwrap message.

Author: beccadax

lib/SILGen/SILGenPoly.cpp

@@ -398,10 +398,11 @@ ManagedValue Transform::transform(ManagedValue v,
   // optional or Any, force it.
   if (inputIsOptional && !outputIsOptional &&
       !outputSubstType->isExistentialType()) {
-    // isImplicitUnwrap is hardcoded false because this method is never
-    // used to generate code from a ForceValueExpr.
+    // isImplicitUnwrap is hardcoded true because the looseness in types of
+    // @objc witnesses/overrides that we're handling here only allows IUOs,
+    // not explicit Optionals.
     v = SGF.emitCheckedGetOptionalValueFrom(Loc, v,
-                                            /*isImplicitUnwrap*/ false, 
+                                            /*isImplicitUnwrap*/ true, 
                                             SGF.getTypeLowering(v.getType()),
                                             SGFContext());