Rollup merge of rust-lang#121366 - nnethercote:rm-diagnostic_builder.rs, r=compiler-errors

Remove `diagnostic_builder.rs`

rust-lang#120576 moved a big chunk of `DiagnosticBuilder`'s functionality out of `diagnostic_builder.rs` into `diagnostic.rs`, which left `DiagnosticBuilder` spread across the two files.

This PR fixes that messiness by merging what remains of `diagnostic_builder.rs` into `diagnostic.rs`.

This is part of rust-lang/compiler-team#722.

r? `@davidtwco`

Author: fmease