[demangler] Fix node matchers

* Add instantiation tests to ItaniumDemangleTest, to make sure all
  match functions provide constructor arguments to the provided functor.

* Fix the Node constructors that lost const qualification on arguments.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D122665

GitOrigin-RevId: abffdd88767791ef6da4d2df7ec7ab158eb8b775