[lld-macho][nfc] rename parsed-section types & variables

This is an NFC diff that prepares for pruning & relocating `__eh_frame`.

Along the way, I made the following changes to ...
* clarify usage of `section` vs. `subsection`
* remove `map` & `vec` from type names
* disambiguate class `Section` from template parameter `SectionHeader`.

Differential Revision: https://reviews.llvm.org/D113241