[flang] Fix handling of elem_len in CFI_establish

The current code computes the minimum element length based on the `type`
used to create the descriptor and uses that as the element length
whenever it is greater than 0. This means that the `elem_len` parameter
is essentially ignored for any type where we can compute a minimum
element length (which includes `CFI_type_char[16|32]_t`), and we may
therefore end up with descriptors with a lower element length than
expected.

This patch fixes the issue by explicitly doing what the standard says,
i.e. it uses the given `elem_len` for character types, `CFI_type_struct`
and `CFI_type_other`, and ignores it (falls back to the minimum element
length) for everything else.

Differential Revision: https://reviews.llvm.org/D101659