Rollup merge of #137213 - nnethercote:rm-rustc_middle-mir-tcx, r=compiler-errors

Remove `rustc_middle::mir::tcx` module.

This is a really weird module. For example, what does `tcx` in `rustc_middle::mir::tcx::PlaceTy` mean? The answer is "not much".

The top-level module comment says:

> Methods for the various MIR types. These are intended for use after
> building is complete.

Awfully broad for a module that has a handful of impl blocks for some MIR types, none of which really relates to `TyCtxt`. `git blame` indicates the comment is ancient, from 2015, and made sense then.

This module is now vestigial. This commit removes it and moves all the code within into `rustc_middle::mir::statement`. Some specifics:

- `Place`, `PlaceRef`, `Rvalue`, `Operand`, `BorrowKind`: they all have `impl` blocks in both the `tcx` and `statement` modules. The commit merges the former into the latter.

- `BinOp`, `UnOp`: they only have `impl` blocks in `tcx`. The commit moves these into `statement`.

- `PlaceTy`, `RvalueInitializationState`: they are defined in `tcx`. This commit moves them into `statement` *and* makes them available in `mir::*`, like many other MIR types.

r? `@tmandry`

compiler/rustc_mir_transform/src/elaborate_drop.rs

@@ -185,7 +185,7 @@ where
             place.ty(self.elaborator.body(), self.tcx()).ty
         } else {
             // We don't have a slice with all the locals, since some are in the patch.
-            tcx::PlaceTy::from_ty(self.elaborator.patch_ref().local_ty(place.local))
+            PlaceTy::from_ty(self.elaborator.patch_ref().local_ty(place.local))
                 .multi_projection_ty(self.elaborator.tcx(), place.projection)
                 .ty
         }