[lld-macho][nfc] Comments and style fixes

Added some comments (particularly around finalize() and
finalizeContents()) as well as doing some rephrasing / grammar fixes for
existing comments.

Also did some minor style fixups, such as by putting methods together in
a class definition and having fields of similar types next to each
other.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D118714