ABI compatible function prototype lowering

[xlauko]

We want to emit ABI compatible low-level dialect. In clang, this is performed before lowering to LLVM. We want to give the same result when lowering from high-level in the low-level dialect. Lowering requires data layout modeling first.

Problems to solve:

• [ ] implement mlir type transformation pass that will split types/merge types/promote to allocas according to ABI rules
• [ ] keep provenance to original type

[lkorenc]

This is a mountain of work, therefore progress will be tracked in separate issues with abi-lowering tag.