Overhaul `Diagnostic` and `DiagnosticBuilder`#120576

Merged

bors merged 2 commits intorust-lang:masterfrom

nnethercote:merge-Diagnostic-DiagnosticBuilder

Feb 20, 2024

Commits on Feb 19, 2024

Prefer `DiagnosticBuilder` over `Diagnostic` in diagnostic modifiers.
nnethercote
committed

Commits on Feb 20, 2024

Reduce capabilities of `Diagnostic`.
nnethercote
committed