[ObjCDirectPreconditionThunk] Setup helper functions (#170617)

## TL;DR

This is a stack of PRs implementing features to expose direct methods
ABI.
You can see the RFC, design, and discussion
[here](https://discourse.llvm.org/t/rfc-optimizing-code-size-of-objc-direct-by-exposing-function-symbols-and-moving-nil-checks-to-thunks/88866).

https://github.com/llvm/llvm-project/pull/170616 Flag
`-fobjc-direct-precondition-thunk` set up
https://github.com/llvm/llvm-project/pull/170617 **Code refactoring to
ease later reviews**
https://github.com/llvm/llvm-project/pull/170618 Thunk generation
https://github.com/llvm/llvm-project/pull/170619 Optimizations, some
class objects can be known to be realized

## Implementation details


1. `GenerateDirectMethodsPreconditionCheck`: Move some functionalities
to a separate functions. Those functions will be reused if we move
precondition checks into a thunk
2. `GenerateClassRealization` separates the class realization `[self
self]` logic into a new function. It comes handy later when we need to
sometimes inline class realization (for va arg functions)
3. Create `DirectMethodInfo`, a two pointer struct that manage true
implementation and its thunk. When `ObjCDirectPreconditionThunk` is not
used, only true implementation is valid pointing to the LLVM IR
Function.

## Tests

None, existing ones shouldn't break