Improve debugDescription of Diagnostic

Since `SourceLocation` is no longer `ExpressibleByDebugDescription` due to the addition of `presumedFile` and `presumedLine`, we need to manually print just line and column here.

Author: ahoppen

Sources/SwiftDiagnostics/Diagnostic.swift

@@ -70,7 +70,7 @@ public struct Diagnostic: CustomDebugStringConvertible {
   public var debugDescription: String {
     let locationConverter = SourceLocationConverter(file: "", tree: node.root)
     let location = location(converter: locationConverter)
-    return "\(location): \(message)"
+    return "\(location.line):\(location.column): \(message)"
   }
 }