Remove `DiagnosticBuilder::into_diagnostic` from `-Ztreat-err-as-bug` consideration.

It seems very wrong to have a `-Ztreat-err-as-bug` check here before the
error is even emitted.

Once that's done:
- `into_diagnostic` is infallible, so its return type doesn't need the
  `Option`;
- the `&'a DiagCtxt` also isn't needed, because only one callsite uses
  it, and it already have access to it via `self.dcx`;
- the comments about dcx disabling buffering are no longer true, this is
  unconditional now;
- and the `debug!` seems unnecessary... the comment greatly overstates
  its importance because few diagnostics come through `into_diagnostic`,
  and `-Ztrack-diagnostics` exists anyway.

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