Switch libbcc to start using MetadataExtractor.

BUG=6051742

This reduces code duplication, and makes us a client of our own helper library.
It also helps to fix a potential issue that can come up with older bitcode
containing only ForEach signatures and not names (something that was fixed in
MetadataExtractor, but not libbcc). We no longer look at metadata nodes at all
from libbcc.

Change-Id: I5cdacec82e4cd164f1aec5aa28a5e82feb9cfc15
7 files changed