[interop][sourcekit] print submodules of CxxStdlib when printing out IDE interface of it

Author: hyp

test/SourceKit/InterfaceGen/gen_clang_libcxx_sdk_module.swift

@@ -0,0 +1,6 @@
+// RUN: %sourcekitd-test -req=interface-gen -module CxxStdlib -- -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import -cxx-interoperability-mode=swift-5.9 -target %target-triple -sdk %sdk | %FileCheck %s
+
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+// CHECK: enum std {
+// CHECK-DAG: public typealias string = std.basic_string<

tools/SourceKit/lib/SwiftLang/SwiftEditorInterfaceGen.cpp

@@ -290,15 +290,18 @@ static bool getModuleInterfaceInfo(ASTContext &Ctx,
 
   PrintOptions Options = PrintOptions::printModuleInterface(
       Ctx.TypeCheckerOpts.PrintFullConvention);
+  ModuleTraversalOptions TraversalOptions = None; // Don't print submodules.
   if (Mod->findUnderlyingClangModule()) {
+    // We need to print the submodules of the C++ standard library.
+    if (ModuleName == "CxxStdlib")
+      TraversalOptions = ModuleTraversal::VisitSubmodules;
     if (Ctx.LangOpts.EnableCXXInterop) {
       // Show unavailable C++ APIs.
       Options.SkipUnavailable = false;
       // Skip over inline namespaces.
       Options.SkipInlineCXXNamespace = true;
     }
   }
-  ModuleTraversalOptions TraversalOptions = None; // Don't print submodules.
   SmallString<128> Text;
   llvm::raw_svector_ostream OS(Text);
   AnnotatingPrinter Printer(Info, OS);