[llvm][ocaml] Replace deprecated C functions in OCaml bindings

Follow-up to D135524, to replace two more deprecated C functions
in the OCaml bindings. const_in_bounds_gep now accepts the source
element type as argument, and const_element has been changed into
aggregate_element, which works on a wider range of constants and
returns an option.

Differential Revision: https://reviews.llvm.org/D136914