[cxx-interop] Disallow import std, require import CxxStdlib

`CxxStdlib` will now be the only accepted module name for the C++ standard library module in Swift.

lib/AST/Module.cpp

@@ -1908,6 +1908,8 @@ StringRef ModuleDecl::ReverseFullNameIterator::operator*() const {
 
   auto *clangModule =
       static_cast<const clang::Module *>(current.get<const void *>());
+  if (!clangModule->isSubModule() && clangModule->Name == "std")
+    return "CxxStdlib";
   return clangModule->Name;
 }