Refactor implicit function template implementation and fix 58534, 58536, 58548, 58549...

Refactor implicit function template implementation and fix 58534, 58536, 58548, 58549 and 58637.

gcc/
	* tree.c (grow_tree_vec_stat): New function ...
	* tree.h (grow_tree_vec_stat) (grow_tree_vec): ... and its declaration
	and macro front-end.

gcc/cp/
	PR c++/58534
	PR c++/58536
	PR c++/58548
	PR c++/58549
	PR c++/58637
	* parser.h (struct cp_parser): New members implicit_template_parms,
	implicit_template_scope and auto_is_implicit_function_template_parm_p.
	* parser.c (add_implicit_template_parms): Refactor as ...
	(synthesize_implicit_template_parm): ... this to append a new template
	type parm to the current template parameter list (introducing a new list
	if necessary).  Removed push_deferring_access_checks.
	(finish_fully_implicit_template): Removed pop_deferring_access_checks.
	(cp_parser_new): Initialize new cp_parser members.
	(cp_parser_parameter_declaration_clause): Consider auto as implicit
	template parm when parsing a parameter declaration (unless parsing an
	explicit specialization).
	(cp_parser_parameter_declaration_list): Remove local
	implicit_template_parms counter and reset cp_parser implicit template
	state when complete.
	(cp_parser_lambda_expression): Reset implicit template cp_parser members
	whilst generating lambda class.
	(cp_parser_function_definition_after_declarator): Reset implicit
	template cp_parser members whilst parsing function definition.
	(make_generic_type_name): Respell '<autoN>' as 'auto:N' which works
	better with template diagnostics.
	(cp_parser_simple_type_specifier): Synthesize implicit template parm on
	parsing 'auto' if auto_is_implicit_function_template_parm_p and provide
	diagnostics ...
	* decl.c (grokdeclarator): ... that were previously done here.

gcc/testsuite/g++.dg/
	* cpp1y/pr58534.C: New testcase.
	* cpp1y/pr58536.C: New testcase.
	* cpp1y/pr58548.C: New testcase.
	* cpp1y/pr58549.C: New testcase.
	* cpp1y/pr58637.C: New testcase.

From-SVN: r204714