[lld-macho] Refactor segment/section creation, sorting, and merging

[lld-macho] Refactor segment/section creation, sorting, and merging

Summary:
There were a few issues with the previous setup:

1. The section sorting comparator used a declarative map of section names to
  determine the correct order, but it turns out we need to match on more than
  just names -- in particular, an upcoming diff will sort based on whether the
  S_ZERO_FILL flag is set. This diff changes the sorter to a more imperative but
  flexible form.

2. We were sorting OutputSections stored in a MapVector, which left the
  MapVector in an inconsistent state -- the wrong keys map to the wrong values!
  In practice, we weren't doing key lookups (only container iteration) after the
  sort, so this was fine, but it was still a dubious state of affairs. This diff
  copies the OutputSections to a vector before sorting them.

3. We were adding unneeded OutputSections to OutputSegments and then filtering
  them out later, which meant that we had to remember whether an OutputSegment
  was in a pre- or post-filtered state. This diff only adds the sections to the
  segments if they are needed.

In addition to those major changes, two minor ones worth noting:

1. I renamed all OutputSection variable names to `osec`, to parallel `isec`.
  Previously we were using some inconsistent combination of `osec`, `os`, and
  `section`.

2. I added a check (and a test) for InputSections with names that clashed with
  those of our synthetic OutputSections.

Reviewers: #lld-macho

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81887