[flang] Use CFI_TYPE_LAST instead of CFI_type_struct

It looks like CFI_type_struct was once used as the last valid CFI_type
value, but in the meantime CFI_type_char16_t and CFI_type_char32_t were
added, making that assumption no longer true. Luckily, in the meantime
we also got a define for CFI_TYPE_LAST, which we can now use to allow
CFI_establish and CFI_allocate to work with descriptors of
CFI_type_char16_t, CFI_type_char32_t and any other future types.

Differential Revision: https://reviews.llvm.org/D101658