Replace the `DiagnosticsEmitter` with a `Diagnostics` type that has direct API for emitting errors, warnings, and remarks.
Replace the `DiagnosticsEmitter` with a `Diagnostics` type that has direct API for emitting errors, warnings, and remarks.

abertelrud/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution