I'm looking at eliminating `DiagnosticConverter`. This change removes
`NodeLocConverter` (albeit adding `UnitAndImportsDiagnosticConverter`),
and in doing so, refactors lex conversion functions to extract them out
from the `DiagnosticConverter` functions.
I'll be following up with changes that collapse `DiagnosticConverter`
logic into `DiagnosticEmitter` locations. The intent is that we
shouldn't need separate ownership of both types.
Jon Ross-Perkins
133717cd7e