SILGen: Fix crash in emitEnumConstructor() if type lowering depends on resilience

It's possible that the result value here is passed indirectly,
but the enum value is loadable inside the constructor's body.
Let's make sure we handle this correctly.

Fixes https://bugs.swift.org/browse/SR-12966 / rdar://problem/70395861.

Author: slavapestov

lib/SILGen/SILGenConstructor.cpp

@@ -544,8 +544,11 @@ void SILGenFunction::emitEnumConstructor(EnumElementDecl *element) {
   // Return the enum.
   auto ReturnLoc = ImplicitReturnLocation::getImplicitReturnLoc(Loc);
 
-  if (mv.isInContext()) {
-    assert(enumTI.isAddressOnly());
+  if (dest) {
+    if (!mv.isInContext()) {
+      dest->copyOrInitValueInto(*this, Loc, mv, /*isInit*/ true);
+      dest->finishInitialization(*this);
+    }
     scope.pop();
     B.createReturn(ReturnLoc, emitEmptyTuple(Loc));
   } else {

test/SILGen/protocol_enum_witness.swift

@@ -17,6 +17,13 @@ public enum AnotherBar: AnotherFoo {
   case bar(arg: Int)
 }
 
+public struct PublicStruct {}
+
+enum InternalEnumWithPublicStruct : Foo {
+  case button
+  case other(PublicStruct)
+}
+
 // CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s21protocol_enum_witness3BarOAA3FooA2aDP6buttonxvgZTW : $@convention(witness_method: Foo) (@thick Bar.Type) -> @out Bar {
 // CHECK: bb0([[BAR:%.*]] : $*Bar, [[BAR_TYPE:%.*]] : $@thick Bar.Type):
 // CHECK-NEXT: [[META_TYPE:%.*]] = metatype $@thin Bar.Type