[BinaryFormat] Add DT_USED tag into dynamic section.

Summary:
This tag is documented in https://docs.oracle.com/cd/E19253-01/817-1984/chapter6-42444/index.html 
Though I could not find some docs that describe it in detail, I found some code snippets.

1.
```
	/*
	 * Look up the string in the string table and get its offset. If
	 * this succeeds, then it is possible that there is a DT_NEEDED
	 * dynamic entry that references it.
	 */
	have_string = elfedit_sec_findstr(argstate->str.sec,
	    strpad_elt.dn_dyn.d_un.d_val, arg, &str_offset) != 0;
	if (have_string) {
		dyn = argstate->dynamic.data;
		for (ndx = 0; ndx < numdyn; dyn++, ndx++) {
			if (((dyn->d_tag == DT_NEEDED) ||
			    (dyn->d_tag == DT_USED)) &&
			    (dyn->d_un.d_val == str_offset))
				goto done;
		}
	}
```
https://github.com/kofemann/opensolaris/blob/80192cd83bf665e708269dae856f9145f7190f74/usr/src/cmd/sgs/elfedit/modules/common/syminfo.c#L512

2.
```
    case DT_USED:
    case DT_INIT_ARRAY:
    case DT_FINI_ARRAY:
      if (do_dynamic)
        {
          if (entry->d_tag == DT_USED
          && VALID_DYNAMIC_NAME (entry->d_un.d_val))
        {
          char *name = GET_DYNAMIC_NAME (entry->d_un.d_val);

          if (*name)
            {
              printf (_("Not needed object: [%s]\n"), name);
              break;
            }
        }

          print_vma (entry->d_un.d_val, PREFIX_HEX);
          putchar ('\n');
        }
      break;
```
http://web.mit.edu/freebsd/head/contrib/binutils/binutils/readelf.c

3.
```
#define DT_USED     0x7ffffffe  /* ignored - same as needed */
```
https://github.com/switchbrew/switch-tools/blob/master/src/elf_common.h

Reviewers: jhenderson, grimar

Reviewed By: jhenderson, grimar

Subscribers: emaste, krytarowski, fedor.sergeev, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58762

llvm-svn: 355468