c++: Prune dead functions.

I was looking at the LCOV coverage report for the C++ FE and
found a bunch of unused functions that I think we can remove.
Obviously, I left alone various dump_* and debug_* routines.
I haven't removed cp_build_function_call although it is also
currently unused.

* lambda_return_type: was used in parser.c in GCC 7, unused since r255950,
* classtype_has_non_deleted_copy_ctor: appeared in GCC 10, its usage
  was removed in c++/95350,
* contains_wildcard_p: used in GCC 9, unused since r276764,
* get_template_head_requirements: seems to never have been used,
* check_constrained_friend: seems to never have been used,
* subsumes_constraints: unused since r276764,
* push_void_library_fn: usage removed in r248328,
* get_template_parms_at_level: unused since r157857,
* get_pattern_parm: unused since r275387.

(Some of the seemingly unused functions, such as set_global_friend, are
actually used in libcc1.)

gcc/cp/ChangeLog:

	* class.c (classtype_has_non_deleted_copy_ctor): Remove.
	* constraint.cc (contains_wildcard_p): Likewise.
	(get_template_head_requirements): Likewise.
	(check_constrained_friend): Likewise.
	(subsumes_constraints): Likewise.
	* cp-tree.h (classtype_has_non_deleted_copy_ctor): Likewise.
	(push_void_library_fn): Likewise.
	(get_pattern_parm): Likewise.
	(get_template_parms_at_level): Likewise.
	(lambda_return_type): Likewise.
	(get_template_head_requirements): Likewise.
	(check_constrained_friend): Likewise.
	(subsumes_constraints): Likewise.
	* decl.c (push_void_library_fn): Likewise.
	* lambda.c (lambda_return_type): Likewise.
	* pt.c (get_template_parms_at_level): Likewise.
	(get_pattern_parm): Likewise.