c++: Check constraints before instantiation from mark_used [PR95132]

c++: Check constraints before instantiation from mark_used [PR95132]

This makes mark_used check constraints of a function _before_ calling
maybe_instantiate_decl, so that we don't try instantiating a function
(as part of return type deduction) with unsatisfied constraints.

gcc/cp/ChangeLog:

	PR c++/95132
	* decl2.c (mark_used): Move up the constraints_satisfied_p check
	so that we check constraints before calling maybe_instantiate_decl.

gcc/testsuite/ChangeLog:

	PR c++/95132
	* g++.dg/cpp2a/concepts-fn7.C: New test.

(cherry picked from commit 9ccc3495766116ea4ae8e4cd8129beca60e30445)