[lld][MachO] Refactor handling of subsections

This diff is a preparation for fixing FunStabs (incorrect size calculation).
std::map<uint32_t, InputSection*> (SubsectionMap) is replaced with
a sorted vector + binary search. If .subsections_via_symbols is set
this vector will contain the list of subsections, otherwise,
the offsets will be used for calculating the symbols sizes.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D98837