Emitter cleanups Some cleanups I made when reading emitter code. In particular, `HumanEmitter` and `JsonEmitter` have gone from three constructors to one. r? `@oli-obk`
rustc::potential_query_instability
DiagnosticArg{,Map,Name,Value}
DiagArg{,Map,Name,Value}
MetadataLoader{,Dyn}
rustc_metadata
DiagnosticBuilder
Diag
NonZero
NonZero::<_>::new
NonZero::new
abort_if_errors
EarlyDiagCtxt
DiagCtxt
JsonEmitter
HumanEmitter
current_rustc_version