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

[Lang Hames via llvm-commits]

https://github.com/lhames approved this pull request.

Hopefully didn't break this too badly with my debugging output changes. If the bots pass I'll merge this and the rest of the comments can be addressed post commit.

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