c++: Fix up handling of register ... asm ("...") vars in templates [PR33661, PR98847]

c++: Fix up handling of register ... asm ("...") vars in templates [PR33661, PR98847]

As the testcase shows, for vars appearing in templates, we don't attach
the asm spec string to the pattern decls, nor pass it back to cp_finish_decl
during instantiation.

The following patch does that.

2021-01-28  Jakub Jelinek  <jakub@redhat.com>

	PR c++/33661
	PR c++/98847
	* decl.c (cp_finish_decl): For register vars with asmspec in templates
	call set_user_assembler_name and set DECL_HARD_REGISTER.
	* pt.c (tsubst_expr): When instantiating DECL_HARD_REGISTER vars,
	pass asmspec_tree to cp_finish_decl.

	* g++.dg/opt/pr98847.C: New test.

(cherry picked from commit cf93f94b3498f3925895fb0bbfd4b64232b9987a)