[Fortran Support] Materialize outermost dimension for Fortran array.

- We use the outermost dimension of arrays since we need this
information to generate GPU transfers.

- In general, if we do not know the outermost dimension of the array
(because the indexing expression is non-affine, for example) then we
simply cannot generate transfer code.

- However, for Fortran arrays, we can use the Fortran array
representation which stores the dimensions of all arrays.

- This patch uses the Fortran array representation to generate code that
computes the outermost dimension size.

Differential Revision: https://reviews.llvm.org/D32967

llvm-svn: 303429