[cxx-interop] Define implict constructor on the definition.

Make sure that we define a C++ record's implicit constructor on the
record's definition, not just on a declaration.

Author: zoecarver

lib/ClangImporter/ImportDecl.cpp

@@ -3539,7 +3539,7 @@ namespace {
           decl->needsImplicitDefaultConstructor()) {
         clang::CXXConstructorDecl *ctor =
             clangSema.DeclareImplicitDefaultConstructor(
-                const_cast<clang::CXXRecordDecl *>(decl));
+                const_cast<clang::CXXRecordDecl *>(decl->getDefinition()));
         if (!ctor->isDeleted())
           clangSema.DefineImplicitDefaultConstructor(clang::SourceLocation(),
                                                      ctor);

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

@@ -58,7 +58,10 @@
 
 // CHECK: struct HasForwardDeclaredNestedType {
 // CHECK:   struct NormalSubType {
+// CHECK:     init()
 // CHECK:   }
 // CHECK:   struct ForwardDeclaredType {
+// CHECK:     init()
 // CHECK:   }
+// CHECK:   init()
 // CHECK: }