Implement constexpr variable templates

	Implement constexpr variable templates
	* decl.c (grokvardecl): Handle specializations of variable templates.
	(grokdeclarator): Handle variable template id expressions and NULL_TREE
	return from grokvardecl.
	* decl2.c (check_member_template): Allow declaration of template member
	variables.
	* parser.c (cp_parser_template_id): Build a TEMPLATE_ID_EXPR for
	variable templates.
	* pt.c (check_template_variable): Accept variable temploids at
	non-class scope.
	(push_template_decl_real): The current instantiation of a template
	can be a VAR_DECL.
	(determine_specialization): Accept variable templates.
	(check_explicit_specialization): Handle and check for malformed
	variable template specializations.
	(lookup_template_variable): New.
	(tsubst_decl): Handle variable template specializations.
	(do_decl_instantiation): Handle template variables.
	(instantiate_decl): Handle template variables.
	* semantics.c (finish_template_variable): New.
	(finish_id_expression): Instantiate variable templates.
	* cp-tree.h (variable_template_p): New.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@213641 138bc75d-0d04-0410-961f-82ee72b054a4