[cxx-interop] Work with definition in VisitCXXRecordDecl.

This was previously un-noticed because VisitRecordDecl already handles this, but there are a couple checks that are specific to C++ records.

Author: zoecarver

lib/ClangImporter/ImportDecl.cpp

@@ -3772,6 +3772,12 @@ namespace {
       if (!Impl.SwiftContext.LangOpts.EnableCXXInterop)
         return VisitRecordDecl(decl);
 
+      decl = decl->getDefinition();
+      if (!decl) {
+        forwardDeclaration = true;
+        return nullptr;
+      }
+
       auto &clangSema = Impl.getClangSema();
       // Make Clang define any implicit constructors it may need (copy,
       // default). Make sure we only do this if the class has been fully defined

test/Interop/Cxx/class/Inputs/forward-declared-cxx-record-used.h

@@ -0,0 +1,8 @@
+#ifndef TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_USED_H
+#define TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_USED_H
+
+struct ForwardDeclaredInOtherHeader;
+
+ForwardDeclaredInOtherHeader *usedInOtherHeader();
+
+#endif // TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_USED_H

test/Interop/Cxx/class/Inputs/forward-declared-cxx-record.h

@@ -0,0 +1,10 @@
+#ifndef TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_H
+#define TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_H
+
+#include "forward-declared-cxx-record-used.h"
+
+struct __attribute__((swift_attr("import_as_ref"))) ForwardDeclaredInOtherHeader {
+  ~ForwardDeclaredInOtherHeader() = delete;
+};
+
+#endif // TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_CXX_RECORD_H

test/Interop/Cxx/class/Inputs/module.modulemap

@@ -97,3 +97,8 @@ module DictionaryOfNSStrings {
   header "dictionary-of-nsstrings.h"
   requires cplusplus
 }
+
+module ForwardDeclaredCxxRecord {
+  header "forward-declared-cxx-record.h"
+  requires cplusplus
+}

test/Interop/Cxx/class/forward-declared-cxx-record-module-interface.swift

@@ -0,0 +1,4 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=ForwardDeclaredCxxRecord -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+
+// CHECK: func usedInOtherHeader() -> ForwardDeclaredInOtherHeader!
+// CHECK: class ForwardDeclaredInOtherHeader