[llvm-objdump] Add RAII for xar apis

Summary:
xar_open and xar_iter_new require manual calls to close/free functions
to deallocate resources. This makes it easy to introduce memory leaks,
so add RAII struct wrappers for these resources.

Reviewers: enderby, rafael, compnerd, lhames, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38598

llvm-svn: 315069