[llvm] [Orc] Move SymbolStringPool from EPC to ExecutionSession (NFC) (PR #85056)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

>From the upstream code-base's point of view this patch makes total sense. For downstream users, the issue is that we don't have an extension point for `ExecutionSession`. Keeping it in `ExecutorProcessControl` allows for better customization. I guess it's one of the things that should be considered in a broader design overhaul at some point.

https://github.com/llvm/llvm-project/pull/85056