llvm-dwarfdump: implement --find for .apple_names

This patch implements the dwarfdump option --find=<name>.  This option
looks for a DIE in the accelerator tables and dumps it if found.  This
initial patch only adds support for .apple_names to keep the review
small, adding the other sections and pubnames support should be
trivial though.

Differential Revision: https://reviews.llvm.org/D38282

llvm-svn: 314439