[flang] Don't implicitly host-associate unknown EQUIVALENCE designators

When resolving names in a specification part, unknown names that appear
in a specification expression before any local declaration are assumed
to be implicitly declared objects in the host scope.  Objects in
EQUIVALENCE sets are not part of specification expressions, so ensure
that they do not receive this treatment; besides being wrong and
unimplementable, it will lead to a later crash during offset assignment.

Differential Revision: https://reviews.llvm.org/D159030