c++: constraint variable used in evaluated context [PR117849]

c++: constraint variable used in evaluated context [PR117849]

Here we wrongly reject the type-requirement at parse time due to its use
of the constraint variable 't' within a template argument (an evaluated
context).  Fix this simply by refining the "use of parameter outside
function body" error path to exclude constraint variables.

PR c++/104255 tracks the same issue for function parameters, but fixing
that would be more involved, requiring changes to the PARM_DECL case of
tsubst_expr.

	PR c++/117849

gcc/cp/ChangeLog:

	* semantics.cc (finish_id_expression_1): Allow use of constraint
	variable outside an unevaluated context.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires41.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 6e973e87e3fec6f33e97edf8fce2fcd121e53961)