[Flang][OpenMP][MLIR] Initial derived type member map support

This patch is one in a series of four patches that seeks to refactor
slightly and extend the current record type map support that was
put in place for Fortran's descriptor types to handle explicit
member mapping for record types at a single level of depth.

For example, the below case where two members of a Fortran
derived type are mapped explicitly:

''''
  type :: scalar_and_array
    real(4) :: real
    integer(4) :: array(10)
    integer(4) :: int
  end type scalar_and_array
  type(scalar_and_array) :: scalar_arr

  !$omp target map(tofrom: scalar_arr%int, scalar_arr%real)
''''

Current cases of derived type mapping left for future work are:
  > explicit member mapping of nested members (e.g. two layers of
     record types where we explicitly map a member from the internal
     record type)
  > Fortran's automagical mapping of all elements and nested elements
     of a derived type
  > explicit member mapping of a derived type and then con...