[lld/mac] Cache file IDs of symbols in emitStabs for faster sorting

This reduces the time emitStabs() takes by about 275ms, or 3% of overall
linking time for the project I'm on. Although the parent function is run in
parallel, it's one of the slowest tasks in that concurrent batch (I have
another optimization for another slow task as well).

Differential Revision: https://reviews.llvm.org/D126785