[cxx-interop] Replace std module name with the new spelling CxxStdlib in the overlay

This is the third step in renaming the C++ stdlib module `std` into `CxxStdlib`.

See #62296.

Author: egorzhdan

stdlib/public/Cxx/std/std.swift

@@ -10,5 +10,5 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_exported import std // Clang module
+@_exported import CxxStdlib // Clang module
 @_exported import Cxx // Swift module