fix #15666: mark file as optional in Diagnostic

src/compiler/types.ts

@@ -3355,9 +3355,9 @@ namespace ts {
     }
 
     export interface Diagnostic {
-        file: SourceFile;
-        start: number;
-        length: number;
+        file: SourceFile | undefined;
+        start: number | undefined;
+        length: number | undefined;
         messageText: string | DiagnosticMessageChain;
         category: DiagnosticCategory;
         code: number;