Refactor rustc_trans::abi not to use LLVM

[arielb1]

Currently, rustc_trans::abi uses LLVM, which makes it hard to use from alternate back-ends like Cranelift. If we refactor it to a separate crate that only relies on ty::layout, that would make life easierr.

cc @eddyb.
cc @camlorn - you might be interested in it.

[ahicks92]

This seems like a good idea but also like a really huge project, so have fun with it.

I'm not actually sure what all that module does. My work barely touched it. I doubt I'll be much use here, but I'll certainly monitor. One thing that would be nice to have (and that this may help with) is a C backend, which would have potential uses for things like integrating with Python (which can only compile C). Maybe someone already did that, though.

[eddyb]

Most of it is in #40658 already, but I want to keep this issue open for tracking the move to a different crate.
I'm pretty sure we can do it by having generics over the Ty in TyLayout (and/or the context type), but keep Layout fully defined outside of librustc.

[eddyb]

This was finished by #50228.