This fixes two issues in our handling of constrained class template
friend declarations.

The first issue is that we fail to set the constraints on the injected
class template declaration during tsubst_friend_class.

The second issue is that the template parameter levels within the parsed
constraints of a class template friend declaration are shifted if the
enclosing class is a template, and this shift leads to spurious
constraint mismatch errors in associate_classtype_constraints if the
friend declaration refers to an already declared class template.

gcc/cp/ChangeLog:

	PR c++/93467
	* constraint.cc (associate_classtype_constraints): If there is a
	discrepancy between the current template depth and the template
	depth of the original declaration, then adjust the template
	parameter depth within the current constraints appropriately.
	* pt.c (tsubst_friend_class): Substitute into and set the
	constraints on the injected declaration.

gcc/testsuite/ChangeLog:

	PR c++/93467
	* g++.dg/cpp2a/concepts-friend6.C: New test.
	* g++.dg/cpp2a/concepts-friend7.C: New test.

(cherry picked from commit 11c7261128ad3ee136508703b20e45cbe04f8dd0)