[flang] Extension to distinguish specific procedures

Allocatable dummy arguments can be used to distinguish
two specific procedures in a generic interface when
it is the case that exactly one of them is polymorphic
or exactly one of them is unlimited polymorphic.  The
standard requires that an actual argument corresponding
to an (unlimited) polymorphic allocatable dummy argument
must also be an (unlimited) polymorphic allocatable, so an
actual argument that's acceptable to one procedure must
necessarily be a bad match for the other.

Differential Revision: https://reviews.llvm.org/D112237