[lld-macho][re-land] Support .subsections_via_symbols

Summary:
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.

Depends on D79668.

Reviewers: ruiu, pcc, MaskRay, smeenai, alexshap, gkm, Ktwu, christylee

Reviewed By: smeenai

Subscribers: thakis, llvm-commits, pcc, ruiu

Tags: #llvm

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