Introduce -brief command line option to llvm-dwarfdump

This patch introduces a new command line option, called brief, to
llvm-dwarfdump.  When -brief is used, the attribute forms for the
.debug_info section will not be emitted to output.

Patch by Spyridoula Gravani!

rdar://problem/21474365
Differential Revision: https://reviews.llvm.org/D33867

llvm-svn: 304844