Merge diagnostic_builder.rs into diagnostic.rs.

Because:
- `diagnostic_builder.rs` is small (282 lines),
- `Diagnostic` and `DiagnosticBuilder` are closely related types, and
- there's already an `impl DiagnosticBuilder` block in `diagnostic.rs`.

At the same time, reorder a few of things already in `diagnostic.rs`,
e.g. move `struct Diagnostic` just before `impl Diagnostic`.

This commit only moves code around. There are no functional changes.

Author: nnethercote