[NFC][OCaml] Simplify llvm_global_initializer using ptr_to_option

[jberdine]

Summary

This diff uses ptr_to_option to convert a nullable C pointer to an
OCaml option instead of the redundant implementation in
llvm_global_initializer.

llvm/bindings/ocaml/llvm/llvm_ocaml.c

 \*===----------------------------------------------------------------------===*/
 
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 #include "llvm-c/Core.h"
 #include "llvm-c/Support.h"
 #include "llvm/Config/llvm-config.h"
 #include "caml/memory.h"
 #include "caml/fail.h"
 #include "caml/callback.h"
 
 #include "llvm_ocaml.h"
 
 value llvm_string_of_message(char* Message) {
   value String = caml_copy_string(Message);
   LLVMDisposeMessage(Message);

 /* llvalue -> unit */
 CAMLprim value llvm_delete_global(LLVMValueRef GlobalVar) {
   LLVMDeleteGlobal(GlobalVar);
   return Val_unit;
 }
 
 /* llvalue -> llvalue option */
 CAMLprim value llvm_global_initializer(LLVMValueRef GlobalVar) {
-  CAMLparam0();
-  LLVMValueRef Init;
-  if ((Init = LLVMGetInitializer(GlobalVar))) {
-    value Option = alloc(1, 0);
-    Field(Option, 0) = (value) Init;
-    CAMLreturn(Option);
-  }
-  CAMLreturn(Val_int(0));
+  return ptr_to_option(LLVMGetInitializer(GlobalVar));
 }
 
 /* llvalue -> llvalue -> unit */
 CAMLprim value llvm_set_initializer(LLVMValueRef ConstantVal,
                                     LLVMValueRef GlobalVar) {
   LLVMSetInitializer(GlobalVar, ConstantVal);
   return Val_unit;
 }