[NFC][OCaml] Resolve const and unsigned compilation warnings

There are a number of compilation warnings regarding disregarding
const qualifiers, and casting between pointers to integer types with
different sign.

The incompatible sign warnings are due to treating the result of
`LLVMGetModuleIdentifier` as `const unsigned char *`, but it is
declared as `const char *`.

The dropped const qualifiers are due to the code pattern
`memcpy(String_val(_),_,_)` which ought to be (following the
implementation of the OCaml runtime)
`memcpy((char *)String_val(_),_,_)`. The issue is that `String_val` is
usually used to get the value of an immutable string. But in the
context of the `memcpy` calls, the string is in the process of being
initialized, so is not yet constant.

Differential Revision: https://reviews.llvm.org/D99392