c++: Fix deprecated attribute handling on templates (PR c++/93228)

As the following testcase shows, when deprecated attribute is on a template,
we'd never print the message if any, because the attribute is not
present on the TEMPLATE_DECL with which warn_deprecated_use is called,
but on its DECL_TEMPLATE_RESULT or its type.

2020-01-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93228
	* parser.c (cp_parser_template_name): Look up deprecated attribute
	in DECL_TEMPLATE_RESULT or its type's attributes.

	* g++.dg/cpp1y/attr-deprecated-3.C: New test.