[llvm] [WebAssembly] Introduce CodeGenPassBuilder (PR #208983)

[Heejin Ahn via llvm-commits]

aheejin wrote:

Ah OK. Didn't mean we need that ASAP. I was mostly asking to create a placeholder TODO for it, in case you forgot (but apparently you didn't). 

>From https://github.com/llvm/llvm-project/pull/209956#issuecomment-5010295583:
> Yeah, responded there. Also, what is the status of GlobalISel in WebAssembly. I was under the impression that it's still off by default/experimental (doesn't mean it doesn't need to be ported and I plan on doing that, just curious).

@QuantumSegfault implemented Wasm GlobalISel, so he should be a good reference. (I'm not very familiar)

https://github.com/llvm/llvm-project/pull/208983