Add missing MIR_get_global_item

[Jasu]

Hi,

MIR_get_global_item was missing a definition, so I added it according to my best guess. That guess might be way off though, since I'm completely new to MIR. I also did not test if it works with forward definitions or re-exports.

[vnmakarov]

Thank you for the pull request. I was not aware that there should be such function (there is no documentation for it in MIR.md). Probably the prototype reflects some my intention to have this function.

As for the pull request, the code is correct but not optimal. The item list can be quite long and we should use the table. I add this function implementation later. Thank you again for letting me know about the missed function.

[Jasu]

The item list can be quite long and we should use the table.

Yeah, I actually tried to use the table first, but found out that item_hash uses the pointer to the string, not the string contents (i.e. the cast-to-uint64 in the following code, (uint64_t) MIR_item_name (NULL, it))):

static htab_hash_t item_hash (MIR_item_t it, void *arg) {
  return mir_hash_finish (
    mir_hash_step (mir_hash_step (mir_hash_init (28), (uint64_t) MIR_item_name (NULL, it)),
                   (uint64_t) it->module));
}

I add this function implementation later.

Thanks! I'll manage until then with my implementation.