c++: give fold expressions a location

c++: give fold expressions a location

This improves diagnostic quality for unsatisfied atomic constraints
that consist of a fold expression, e.g. in concepts/diagnostic3.C
the "evaluated to false" diagnostic now points to the expression:

  .../diagnostic3.C:10:22: note: the expression ‘(foo<Ts> && ...) [with Ts = {int, char}]’ evaluated to ‘false’
     10 | requires (foo<Ts> && ...)
        |          ~~~~~~~~~~~~^~~~

gcc/cp/ChangeLog:

	* semantics.cc (finish_unary_fold_expr): Use input_location
	instead of UNKNOWN_LOCATION.
	(finish_binary_fold_expr): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic3.C: Adjusted expected location of
	"evaluated to false" diagnostics.