c++: unique friend shenanigans [PR69836]

c++: unique friend shenanigans [PR69836]

Normally we re-instantiate a function declaration when we start to
instantiate the body in case of multiple declarations.  In this wacky
testcase, this causes a problem because the type of the w_counter parameter
depends on its declaration not being in scope yet, so the name lookup only
finds the previous declaration.  This isn't a problem for member functions,
since they aren't subject to argument-dependent lookup.  So let's just skip
the regeneration for hidden friends.

	PR c++/69836

gcc/cp/ChangeLog:

	* pt.cc (regenerate_decl_from_template): Skip unique friends.

gcc/testsuite/ChangeLog:

	* g++.dg/template/friend76.C: New test.