`DiagnosticSeverity` should conform to `Equatable`

[grynspan]

Description

DiagnosticSeverity has a clearly-defined ranking and should conform to Comparable.

It would make it easier for macro code to e.g. suppress its own warnings (with something like diagnostics.filter { $0.severity >= .error }) or enforce -Werror-like semantics, that sort of thing.

[ahoppen]

Synced to Apple’s issue tracker as rdar://144611943

[ahoppen]

I disagree that DiagnosticSeverity has a clearly defined ranking. error is obviously more severe than warning but there isn’t really a clear ranking between remark and note, they are just different kinds of diagnostics. I thus don’t think that DiagnosticSeverity should be Comparable. Also, if you want to suppress warnings, checking for == .warning or == .error should work equally well as >= .someSeverityLevel.

[grynspan]

It doesn't conform to Equatable either, FWIW.

[ahoppen]

Good point, adding there Equatable conformance here: #2965