Overhaul Diagnostic and DiagnosticBuilder

[nnethercote]

Implements the first part of rust-lang/compiler-team#722, which moves functionality and use away from Diagnostic, onto DiagnosticBuilder.

Likely follow-ups:

• Move things around, because this PR was written to minimize diff size, so some things end up in sub-optimal places. E.g. DiagnosticBuilder has impls in both diagnostic.rs and diagnostic_builder.rs.
• Rename Diagnostic as DiagInner and DiagnosticBuilder as Diag.

r? @davidtwco