When cp_unevaluated_operand is set, tsubst_decl thinks that if it sees a
PARM_DECL that isn't already in local_specializations, we're in a decltype
in a trailing return type or some such, and so we only want a substitution
for a single PARM_DECL.  In this case, we want the whole chain, so make sure
cp_unevaluated_operand is cleared.

gcc/cp/ChangeLog
2020-03-14  Jason Merrill  <jason@redhat.com>

	PR c++/93248
	* pt.c (build_deduction_guide): Clear cp_unevaluated_operand for
	substituting DECL_ARGUMENTS.