[llvm] [ORC] Switch to singleton pattern for UnwindInfoManager. (PR #126691)

[Lang Hames via llvm-commits]

lhames wrote:

> FYI fixed the unused `AddFnName` warning: [9f61a60](https://github.com/llvm/llvm-project/commit/9f61a60c777465c8a1bb67f80560a9e3b4d0f05b)
> 
> It's a bit weird that we only use the `RemoveFnName` when `__APPLE__`, but use it for a debug message in the other case as well.

It's not immediately apparent from the code, but that destructor can only run if the code under the `__APPLE__` guard runs, so it's implicitly guarded too. I'll look at expanding the guards so that we can compile UnwindInfoManager out entirely on non-Apple platforms.

https://github.com/llvm/llvm-project/pull/126691