Add "remark" diagnostics

The Swift compiler (and most LLVM-based tools) have a "remark" diagnostic
severity. Add support for it into DiagnosticSeverity.

Author: DougGregor

Sources/SwiftCompilerPluginMessageHandling/Diagnostics.swift

@@ -53,6 +53,7 @@ extension PluginMessage.Diagnostic.Severity {
     case .error: self = .error
     case .warning: self = .warning
     case .note: self = .note
+    case .remark: self = .remark
     }
   }
 }

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -155,6 +155,7 @@ public enum PluginMessage {
       case error
       case warning
       case note
+      case remark
     }
     public struct Position: Codable {
       public var fileName: String

Sources/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -341,6 +341,10 @@ public struct DiagnosticsFormatter {
     case .note:
       severityText = "note"
       severityAnnotation = .noteText
+
+    case .remark:
+      severityText = "remark"
+      severityAnnotation = .remarkText
     }
 
     let prefix = colorizeIfRequested("\(severityText): ", annotation: severityAnnotation)
@@ -439,4 +443,8 @@ struct ANSIAnnotation {
   static var noteText: ANSIAnnotation {
     ANSIAnnotation(color: .default, trait: .bold)
   }
+
+  static var remarkText: ANSIAnnotation {
+    ANSIAnnotation(color: .blue, trait: .bold)
+  }
 }

Sources/SwiftDiagnostics/Message.swift

@@ -30,6 +30,7 @@ public enum DiagnosticSeverity {
   case error
   case warning
   case note
+  case remark
 }
 
 /// Types conforming to this protocol represent diagnostic messages that can be