[lld-macho] Support .subsections_via_symbols

This diff restores and builds upon @pcc and @ruiu's initial work on
subsections.

The .subsections_via_symbols directive indicates we can split each
section along symbol boundaries, unless those symbols have been marked
with `.alt_entry`.

We exercise this functionality in our tests by using order files that
rearrange those symbols.

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D79926