[lld-macho][nfc] Remove indirection when looking up common section members

{D118797} means that we can now check the name/segname of a given
section directly, instead of having to look those properties up on one
of its subsections. This allows us to simplify our code.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D123275