[lld-macho] Support -order_file
The order file indicates how input sections should be sorted within each
output section, based on the symbols contained within those sections.
This diff sets the stage for implementing and testing
.subsections_via_symbols, where we will break up InputSections by each
symbol and sort them more granularly.
Reviewed By: smeenai
Differential Revision: https://reviews.llvm.org/D79668