[flang] Symbol representation for dummy SubprogramDetails

Dummy procedures can be defined as subprograms with explicit
interfaces, e.g.

  subroutine subr(dummy)
    interface
      subroutine dummy(x)
        real :: x
      end subroutine
    end interface
    ! ...
  end subroutine

but the symbol table had no means of marking such symbols as dummy
arguments, so predicates like IsDummy(dummy) would fail.  Add an
isDummy_ flag to SubprogramNameDetails, analogous to the corresponding
flag in EntityDetails, and set/test it as needed.

Differential Revision: https://reviews.llvm.org/D106697