Replace it with ConstUsize instead, which is more appropriate; we are not using the rest of the
TypedConstVal anyway

Fixes #36082.

This is a clearer name since they represent [a, b, c] array literals.

This implements RFC 1624, tracking issue #37339.

- `FnCtxt` (in typeck) gets a stack of `LoopCtxt`s, which store the
  currently deduced type of that loop, the desired type, and a list of
  break expressions currently seen. `loop` loops get a fresh type
  variable as their initial type (this logic is stolen from that for
  arrays). `while` loops get `()`.
- `break {expr}` looks up the broken loop, and unifies the type of
  `expr` with the type of the loop.
- `break` with no expr unifies the loop's type with `()`.
- When building MIR, `loop` loops no longer construct a `()` value at
  termination of the loop; rather, the `break` expression assigns the
  result of the loop. `while` loops are unchanged.
- `break` respects contexts in which expressions may not end with braced
  blocks. That is, `while break { break-value } { while-body }` is
  illegal; this preserves backwards compatibility.
- The RFC did not make it clear, but I chose to make `break ()` inside
  of a `while` loop illegal, just in case we wanted to do anything with
  that design space in the future.

This is my first time dealing with this part of rustc so I'm sure
there's plenty of problems to pick on here ^_^

This moves closures to the (DefId, Substs) scheme like all other items,
and saves a word from the size of TyS now that Substs is 2 words.

that made no sense (see test), and was incompatible with borrowck.

Fixes #36936.

Split Ty::is_empty method into is_never and is_uninhabited

Replace FnOutput with Ty
Replace FnConverging(ty) with ty
Purge FnDiverging, FunctionRetTy::NoReturn and FunctionRetTy::None