We will shortly refactor things so that it is no longer needed

We store the obligation that caused the overflow as part of the OverflowError, and report it at the public API endpoints (rather than in the implementation internals).

- `ParamEnv::empty()` -- does not reveal all, good for typeck
- `ParamEnv::reveal_all()` -- does, good for trans
- `param_env.with_reveal_all()` -- converts an existing parameter environment

Improvements:

- Use Clone not Copy for the "simple cases"
- Separate TypeFoldable and Lift for the "simple cases"
- Support generics type parameters
- Support named fields in enum variants
- etc

Type-live-for-region obligations on DUMMY_NODE_ID cause an ICE, and it
turns out that in the few cases they are needed, these obligations are not
needed anyway because they are verified elsewhere.

Fixes #46069.

cc #37166

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

Fixes #28229.
Fixes #24000.

The field init shorthand syntax was stabilized in 1.17.0 (aebd94fd); we
are now free to use it in the compiler.

- Point out the origin of a type requirement when it is the return type
  of a method
- Point out possibly missing semicolon when the return type is () and
  the implicit return makes sense as a statement
- Suggest changing the return type of methods with default return type
- Don't suggest changing the return type on fn main()
- Don't suggest changing the return type on impl fn