[flang] Allow for host association in ConvertTo(Object|Proc)Entity

Name resolution fails with a bogus "is not a variable" error message
when a host-associated object appears in a NAMELIST group.  The root
cause is that ConvertToObjectEntity() returns false for host-associated
objects.  Fix that, and also apply a similar fix to ConvertToProcEntity()
nearby.

Differential Revision: https://reviews.llvm.org/D124541