[lld-macho][nfc] Use includeInSymtab for all symtab-skipping logic

{D123302} got me looking deeper at `includeInSymtab`. I thought it was a
little odd that there were excluded (live) symbols for which
`includeInSymtab` was false; we shouldn't have so many different ways to
exclude a symbol. As such, this diff makes the `L`-prefixed-symbol
exclusion code use `includeInSymtab` too. (Note that as part of our
support for `__eh_frame`, we will also be excluding all `__eh_frame`
symbols from the symtab in a future diff.)

Another thing I noticed is that the `emitStabs` code never has to deal
with excluded symbols because `SymtabSection::finalize()` already
filters them out. As such, I've updated the comments and asserts from
{D123302} to reflect this.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D123433