Every `Body` should remember its `MirSource`

[ecstatic-morse]

We have a type called MirSource that is needed for debug logging during the MIR transform pipeline. We always know the source when building a Body, but for some reason we drop it on the floor. It then gets recomputed in rustc_mir and passed around as a separate parameter. This is unergonomic and increases function signature complexity. We should store a MirSource as part of the Body instead.

cc @rust-lang/wg-mir-opt