Allow MachineFunction to obtain non-const Function (to enable MIR-level debugify)

Summary:
To debugify MIR, we need to be able to create metadata and to do that, we
need a non-const Module. However, MachineFunction only had a const reference
to the Function preventing this.

Reviewers: aprantl, bogner

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77439