[IRGen] Emit lazy opaque type descriptors.

These were recorded but never emitted. Fixes rdar://problem/50258536.

Author: DougGregor

lib/IRGen/GenDecl.cpp

@@ -1082,6 +1082,7 @@ void IRGenerator::emitTypeMetadataRecords() {
 void IRGenerator::emitLazyDefinitions() {
   while (!LazyTypeMetadata.empty() ||
          !LazyTypeContextDescriptors.empty() ||
+         !LazyOpaqueTypeDescriptors.empty() ||
          !LazyFieldDescriptors.empty() ||
          !LazyFunctionDefinitions.empty() ||
          !LazyWitnessTables.empty()) {
@@ -1106,6 +1107,11 @@ void IRGenerator::emitLazyDefinitions() {
       emitLazyTypeContextDescriptor(*IGM.get(), type,
                                     RequireMetadata_t(entry.IsMetadataUsed));
     }
+    while (!LazyOpaqueTypeDescriptors.empty()) {
+      auto type = LazyOpaqueTypeDescriptors.pop_back_val();
+      CurrentIGMPtr IGM = getGenModule(type->getDeclContext());
+      IGM->emitOpaqueTypeDecl(type);
+    }
     while (!LazyFieldDescriptors.empty()) {
       NominalTypeDecl *type = LazyFieldDescriptors.pop_back_val();
       CurrentIGMPtr IGM = getGenModule(type->getDeclContext());

test/IRGen/lazy_opaque_result_type.swift

@@ -0,0 +1,26 @@
+// RUN: %target-swift-frontend -parse-as-library -module-name=test -O -primary-file %s -emit-ir > %t.ll
+// RUN: %FileCheck %s < %t.ll
+
+protocol P { }
+
+protocol Q {
+  associatedtype AT: P
+  func getAT() -> AT
+}
+
+private struct X: Q {
+  private struct Inner: P { }
+
+  // CHECK: @"$s4test1X33_58D62B15E5EAC1447430E52C74EAD489LLV5getATQryFQOMQ" = internal constant
+  func getAT() -> some P {
+    return Inner()
+  }
+}
+
+func testMe<T: Q>(_ t: T) {
+  _ = t.getAT()
+}
+
+func doIt() {
+  testMe(X())
+}