Enzyme is not aware of Fortran descriptors

Flang represents allocatable arrays using descriptor structs containing both differentiable and non-differentiable metadata but Enzyme isn't aware of this distinction. This means that it currently tries to differentiate with respect to implicit integer arguments that come with 'assumed shape' array arguments. This is the deeper issue underpinning #2820.

Making Enzyme aware of Fortran descriptor information would likely be greatly beneficial for improving the user experience in Fortran.