[llvm-objcopy] Adding support for decompressing zlib compressed dwarf sections.

Summary: I had added support for compressing dwarf sections in a prior commit,
         this one adds support for decompressing. Usage is:

         llvm-objcopy --decompress-debug-sections input.o output.o

Reviewers: jakehehrlich, jhenderson, alexshap	

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D51841

llvm-svn: 343451