This commit replaces this pattern:
```
err.into_diagnostic(dcx)
```
with this pattern:
```
dcx.create_err(err)
```
in a lot of places.

It's a little shorter, makes the error level explicit, avoids some
`IntoDiagnostic` imports, and is a necessary prerequisite for the next
commit which will add a `level` arg to `into_diagnostic`.

This requires adding `track_caller` on `create_err` to avoid mucking up
the output of `tests/ui/track-diagnostics/track4.rs`. It probably should
have been there already.

Presumably these are a hangover from an earlier time when they were
necessary.

First, it is parameterized by the name of the diagnostic and the
DiagCtxt. These are given to `session_diagnostic_derive` and
`lint_diagnostic_derive`. But the names are hard-wired as "diag" and
"handler" (should be "dcx"), and there's no clear reason for the
parameterization. So this commit removes the parameterization and
hard-wires the names internally.

Once that is done `DiagnosticDeriveBuilder` is reduced to a trivial
wrapper around `DiagnosticDeriveKind`, and can be removed.

Also, `DiagnosticDerive` and `LintDiagnosticDerive` don't need the
`builder` field, because it has been reduced to a kind, and they know
their own kind. This avoids the need for some
`let`/`else`/`unreachable!` kind checks

And `DiagnosticDeriveVariantBuilder` no longer needs a lifetime, because
the `parent` field is changed to `kind`, which is now a trivial copy
type.

And make all hand-written `IntoDiagnostic` impls generic, by using
`DiagnosticBuilder::new(dcx, level, ...)` instead of e.g.
`dcx.struct_err(...)`.

This means the `create_*` functions are the source of the error level.
This change will let us remove `struct_diagnostic`.

Note: `#[rustc_lint_diagnostics]` is added to `DiagnosticBuilder::new`,
it's necessary to pass diagnostics tests now that it's used in
`into_diagnostic` functions.

`EmissionGuarantee` no longer determines the error level, the `create_*`
functions do.

This lets different error levels share the same return type from
`emit_*`.

- A lot of inconsistencies in the `DiagCtxt` API are removed.
- `Noted` is removed.
- `FatalAbort` is introduced for fatal errors (abort via `raise`),
  replacing the `EmissionGuarantee` impl for `!`.
- `Bug` is renamed `BugAbort` (to avoid clashing with `Level::Bug` and
  to mirror `FatalAbort`), and modified to work in the new way with bug
  errors (abort via panic).
- Various diagnostic creators and emitters updated to the new, better
  signatures. Note that `DiagCtxt::bug` no longer needs to call
  `panic_any`, because `emit` handles that.

Also shorten the obnoxiously long
`diagnostic_builder_emit_producing_guarantee` name.

This makes `DiagCtxt::bug` look like the other similar functions.

The easter egg ICE on `break rust` is weird: it's the one ICE in the
entire compiler that doesn't immediately abort, which makes it
annoyingly inconsistent.

This commit changes it to abort. As part of this, the extra notes are
now appended onto the bug dignostic, rather than being printed as
individual note diagnostics, which changes the output format a bit.
These changes don't interferes with the joke, but they do help with my
ongoing cleanups to error handling.