[lld-macho][re-land] Support .subsections_via_symbols
The .subsections_via_symbols directive indicates we can split each
section along symbol boundaries, unless those symbols have been marked
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
Differential Revision: https://reviews.llvm.org/D79926