[llvm] Move a lot of symbol code to use the symbol string pool (PR #115796)

[Lang Hames via llvm-commits]

https://github.com/lhames commented:

This is looking great!

I've suggested a bunch of changes, mostly `std::move` use and other minor stuff. There are just a couple of correctness fixes. I'm ok with this landing as soon as the correctness fixes are done. The cosmetic / perf changes can happen before or after this lands, but we should land it ASAP to avoid bitrot, since this touches a lot of code -- if it'll be a while before you can address all the other comments then let's merge and deal with the other stuff post-commit.

https://github.com/llvm/llvm-project/pull/115796