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.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -18,12 +18,11 @@ use rustc_hir::intravisit::{Visitor, walk_block, walk_expr};
 use rustc_hir::{CoroutineDesugaring, CoroutineKind, CoroutineSource, LangItem, PatField};
 use rustc_middle::bug;
 use rustc_middle::hir::nested_filter::OnlyBodies;
-use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
     self, AggregateKind, BindingForm, BorrowKind, ClearCrossCrate, ConstraintCategory,
     FakeBorrowKind, FakeReadCause, LocalDecl, LocalInfo, LocalKind, Location, MutBorrowKind,
-    Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
-    TerminatorKind, VarBindingForm, VarDebugInfoContents,
+    Operand, Place, PlaceRef, PlaceTy, ProjectionElem, Rvalue, Statement, StatementKind,
+    Terminator, TerminatorKind, VarBindingForm, VarDebugInfoContents,
 };
 use rustc_middle::ty::print::PrintTraitRefExt as _;
 use rustc_middle::ty::{

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -13,10 +13,9 @@ use rustc_infer::infer::{
 };
 use rustc_infer::traits::SelectionError;
 use rustc_middle::bug;
-use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
     AggregateKind, CallSource, ConstOperand, ConstraintCategory, FakeReadCause, Local, LocalInfo,
-    LocalKind, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement,
+    LocalKind, Location, Operand, Place, PlaceRef, PlaceTy, ProjectionElem, Rvalue, Statement,
     StatementKind, Terminator, TerminatorKind, find_self_call,
 };
 use rustc_middle::ty::print::Print;