<rdar://problem/14496092>

Fixes from code review by Jim Ingham that reinstate preferring an external vs non-external symbol when finding function addresses.
Revert "[PECOFF] Add atoms to the PassManager file"

This reverts commit r193479.

The atoms are already added to the file, so re-adding them caused the YAML
writer to write the same atoms twice. That made the YAML reader to fail with
"duplicate atom name" error.

This is not the only error we've got for RoundTripYAMLPass for PECOFF, so we
cannot enable the test yet. More fixes will come.

Differential Revision: http://llvm-reviews.chandlerc.com/D2069