[llvm-dwarfdump] Brief mode only dumps debug_info by default

This patch changes the default behavior in brief mode to only show the
debug_info section. This is undoubtedly the most popular and likely the
one you'd want in brief mode.

Non-brief mode behavior is not affected and still defaults to all.

Differential revision: https://reviews.llvm.org/D37334

llvm-svn: 312252