c++: Respect current_constraint_diagnosis_depth in diagnose_compound_requirement

c++: Respect current_constraint_diagnosis_depth in diagnose_compound_requirement

The previous patch tries to avoid changing our current default diagnostics.  But
for the sake of consistency we arguably should also respect
current_constraint_diagnosis_depth in diagnose_compound_requirement() like we do
in the other error-replaying diagnostic routines.  But doing so would be a
change to our default diagnostics behavior, so the change has been split out
into this separate patch for separate consideration.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_compound_requirement): When diagnosing a
	compound requirement, maybe replay the satisfaction failure, subject to
	the current diagnosis depth.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic1.C: Pass -fconcepts-diagnostics-depth=2.
	* g++.dg/concepts/diagnostic5.C: Adjust expected diagnostics.
	* g++.dg/cpp2a/concepts-iconv1.C: Pass -fconcepts-diagnostics-depth=2.
	* g++.dg/cpp2a/concepts-requires5.C: Likewise.