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.