[llvm] [MC] Eliminate two symbol-related hash maps (PR #95464)

[via llvm-commits]

aengelke wrote:

> How about making the following change then removing the Entry parameter from createSymbol?

This doesn't work that easily, as private user-visible symbols do get renamed. I refactored the code a bit so that the method is now named `createRenamableSymbol`. For user-visible symbols that occur more than once (unlikely to happen), we now do two lookups, but I think that's tolerable.

https://github.com/llvm/llvm-project/pull/95464