MC: Remove last use of MCSymbolData::getSymbol(), NFC

Remove the last use of `MCSymbolData::getSymbol()`.  There's some
*really* hairy stuff going on in `MachObjectWriter::WriteNList()` that I
want to come back to.  In particular, it updates `Symbol` to point at
its aliasee (if any), but leaves `Data` behind, and it's not clear
whether everything makes sense there.

For now I've left the logic unchanged by adding `OrigSymbol` and moving
the FIXME from r237750 up a bit higher.  I've filed PR23598 to track
looking into this.

llvm-svn: 237867