[llvm-dwarfdump] - Teach dwarfdump to decompress debug sections in zlib style.

Before this llvm-dwarfdump only recognized zlib-gnu compression style of headers,
this patch adds support for zlib style. 
It looks reasonable to support both styles for dumping,
even if we are not going to suport generating of deprecated gnu one.

Differential revision: http://reviews.llvm.org/D20470

llvm-svn: 270540