c++: base template friend [PR52625]

Here we were mistakenly treating the injected-class-name as a partial
specialization.

gcc/cp/ChangeLog:

	PR c++/52625
	* pt.c (maybe_process_partial_specialization): Check
	DECL_SELF_REFERENCE_P.

gcc/testsuite/ChangeLog:

	PR c++/52625
	* g++.dg/template/friend70.C: New test.