[ClangImporter] Fix CxxStdlib submodule names in swift interface

A previous commit resulted in the swift interface for CxxStdlib
containing an `import CxxStdlib` for each submodule. This restores the
submodule names to `CxxStdlib.vector` etc.

Author: hnrklssn

lib/ClangImporter/ClangImporter.cpp

@@ -8770,11 +8770,12 @@ bool importer::isCxxStdModule(StringRef moduleName, bool IsSystem) {
 }
 
 ImportPath::Builder importer::getSwiftModulePath(ASTContext &SwiftContext, const clang::Module *M) {
-  if (isCxxStdModule(M))
-    return ImportPath::Builder(SwiftContext.Id_CxxStdlib);
   ImportPath::Builder builder;
   while (M) {
-    builder.push_back(SwiftContext.getIdentifier(M->Name));
+    if (!M->isSubModule() && isCxxStdModule(M))
+      builder.push_back(SwiftContext.Id_CxxStdlib);
+    else
+      builder.push_back(SwiftContext.getIdentifier(M->Name));
     M = M->Parent;
   }
   std::reverse(builder.begin(), builder.end());