[SILGen] Handle default initialization of init accessor in memberwise init

If the property isn't memberwise initializable we need to emit its
initial value and call the init accessor. This currently isn't necessary
since all init accessor properties are included in the memberwise
initializer, but the following commit will change this.

Author: hamishknight

lib/SILGen/SILGenConstructor.cpp

@@ -428,6 +428,18 @@ static void emitImplicitValueConstructor(SILGenFunction &SGF,
                                   selfIfaceTy, selfTy, std::move(*elti));
           ++elti;
           continue;
+        } else {
+          // If the property isn't memberwise initialized, emit the initial
+          // value directly.
+          // TODO: This matches the logic below, but should we be calling
+          // the initializer generator instead to match what we do for
+          // `emitMemberInitializationViaInitAccessor`?
+          auto *init = field->getParentExecutableInitializer();
+          ASSERT(init);
+          FullExpr scope(SGF.Cleanups, field->getParentPatternBinding());
+          auto rvalue = SGF.emitRValue(init);
+          emitApplyOfInitAccessor(SGF, Loc, initAccessor, resultSlot,
+                                  selfIfaceTy, selfTy, std::move(rvalue));
         }
       }