c++: improve class NTTP object pretty printing [PR111471]

c++: improve class NTTP object pretty printing [PR111471]

1. Move class NTTP object pretty printing to a more general spot in
   the pretty printer, so that we always print its value instead of
   its (mangled) name even when it appears outside of a template
   argument list.
2. Print the type of an class NTTP object alongside its CONSTRUCTOR
   value, like dump_expr would have done.
3. Don't print const VIEW_CONVERT_EXPR wrappers for class NTTPs.

	PR c++/111471

gcc/cp/ChangeLog:

	* cxx-pretty-print.cc (cxx_pretty_printer::expression)
	<case VAR_DECL>: Handle class NTTP objects by printing
	their type and value.
	<case VIEW_CONVERT_EXPR>: Strip const VIEW_CONVERT_EXPR
	wrappers for class NTTPs.
	(pp_cxx_template_argument_list): Don't handle class NTTP
	objects here.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic19.C: New test.