[embedded] Avoid compiler crash when using an opaque result type

Author: kubamracek

lib/IRGen/GenStruct.cpp

@@ -1636,6 +1636,9 @@ void IRGenModule::emitStructDecl(StructDecl *st) {
 }
 
 void IRGenModule::maybeEmitOpaqueTypeDecl(OpaqueTypeDecl *opaque) {
+  if (opaque->getASTContext().LangOpts.hasFeature(Feature::Embedded))
+    return;
+
   if (!opaque->isAvailableDuringLowering())
     return;
 

test/embedded/opaque-return-types.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-emit-ir %s -enable-experimental-feature Embedded | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: VENDOR=apple
+// REQUIRES: OS=macosx
+
+protocol Proto { }
+
+struct MyStruct: Proto { }
+
+func foo() -> some Proto {
+  MyStruct()
+}
+
+// CHECK: define {{.*}}@main(