Allow non-constexpr variable templates.

Allow non-constexpr variable templates.

	* decl2.c (note_variable_template_instantiation): New.
	* cp-tree.h: Declare it.
	* pt.c (instantiate_decl): Call it.
	(push_template_decl_real): Allow non-constexpr variable templates.
	* semantics.c (finish_id_expression): Mark the variable template
	instantiation as used.
	* mangle.c (write_mangled_name): Variable template instantiations
	are mangled.
	* parser.c (cp_parser_init_declarator): Complain about
	non-function implicit templates.

From-SVN: r214396