c++: Improve handling of ill-formed constraints [PR94186].

It would have been trivial to make the error for non-bool constraint in
satisfy_atom unconditional, but that didn't give context for the error or
printing with the dependent form and template arguments.  So I changed a
couple of places so that, when a hard error is encountered during quiet
substitution/satisfaction, we go through again noisily; this builds up the
necessary context.

The similar change to tsubst_nested_requirement does not build up the
necessary context; rather than try to fix that now I changed
get_constraint_error_location to give up and use input_location if there's
no CONSTR_CONTEXT.  In the case of concepts-pr67697.C, we still have a good
source location because the NESTED_REQ has a correct EXPR_LOCATION, but this
patch doesn't improve context printing for this case as it does for the
above.

gcc/cp/ChangeLog
2020-03-24  Jason Merrill  <jason@redhat.com>

	PR c++/94186
	* constraint.cc (constraint_satisfaction_value): Repeat noisily on
	error.
	(tsubst_nested_requirement): Likewise.
	(get_constraint_error_location): Allow missing context.
	(diagnose_atomic_constraint): Diagnose non-bool constraint here.
	(satisfy_atom): Not here.  Only diagnose non-constant when noisy.

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