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.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 9cec8d832dd..45fcec41fde 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1125,7 +1125,7 @@ pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>;
 ///
 ///  1. The address in memory that the place refers to.
 ///  2. The provenance with which the place is being accessed.
-///  3. The type of the place and an optional variant index. See [`PlaceTy`][super::tcx::PlaceTy].
+///  3. The type of the place and an optional variant index. See [`PlaceTy`][super::PlaceTy].
 ///  4. Optionally, some metadata. This exists if and only if the type of the place is not `Sized`.
 ///
 /// We'll give a description below of how all pieces of the place except for the provenance are