c++: mangling template-id of unknown template [PR110524]

This fixes a crash when mangling an ADL-enabled call to a template-id
naming an unknown template (as per P0846R0).

	PR c++/110524

gcc/cp/ChangeLog:

	* mangle.cc (write_expression): Handle TEMPLATE_ID_EXPR
	whose template is already an IDENTIFIER_NODE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/fn-template26.C: New test.

(cherry picked from commit 97ceaa11)