[cxx-interop] Skip forward-declared nested structs.

This prevents us from accidentially adding the same sub-type twice.

Author: zoecarver

lib/ClangImporter/ImportDecl.cpp

@@ -3374,6 +3374,11 @@ namespace {
         }
 
         if (isa<TypeDecl>(member)) {
+          // Only import definitions. Otherwise, we might add the same member
+          // twice.
+          if (auto tagDecl = dyn_cast<clang::TagDecl>(nd))
+            if (tagDecl->getDefinition() != tagDecl)
+              continue;
           // A struct nested inside another struct will either be logically
           // a sibling of the outer struct, or contained inside of it, depending
           // on if it has a declaration name or not.

test/Interop/Cxx/class/Inputs/nested-records.h

@@ -45,6 +45,12 @@ struct S10 {
   };
 };
 
+struct HasForwardDeclaredNestedType {
+  struct ForwardDeclaredType;
+  struct NormalSubType { };
+  struct ForwardDeclaredType { };
+};
+
 // TODO: Nested class templates (SR-13853).
 
 #endif // TEST_INTEROP_CXX_CLASS_INPUTS_NESTED_RECORDS_H

test/Interop/Cxx/class/nested-records-module-interface.swift

@@ -55,3 +55,10 @@
 // CHECK:     }
 // CHECK:   }
 // CHECK: }
+
+// CHECK: struct HasForwardDeclaredNestedType {
+// CHECK:   struct NormalSubType {
+// CHECK:   }
+// CHECK:   struct ForwardDeclaredType {
+// CHECK:   }
+// CHECK: }