[llvm] [CodeGen][Pass] Add `TargetPassBuilder` (PR #137290)

[via llvm-commits]

paperchalice wrote:

> What exactly do you consider the hack here? The logic in `constructRealPassManager()` with respect to the explicit nesting requirement? Or do you mean the hijacking of the pass-managers for the intermediate structure?

Shadowing `llvm::PassManager` with `llvm::TargetPassBuilder::PassManager`, because I want to minimize the differences in usage between the two manager. Fortunately, it doesn't trigger warnings from `-Wshadow`.

https://github.com/llvm/llvm-project/pull/137290