Cache the latest source file from document registry so we can keep it alive when script info is orphan

src/server/scriptInfo.ts

@@ -202,6 +202,12 @@ namespace ts.server {
         return fileName[0] === "^" || getBaseFileName(fileName)[0] === "^";
     }
 
+    /*@internal*/
+    export interface DocumentRegistrySourceFileCache {
+        key: DocumentRegistryBucketKey;
+        sourceFile: SourceFile;
+    }
+
     export class ScriptInfo {
         /**
          * All projects that include this file
@@ -221,6 +227,9 @@ namespace ts.server {
         /** Set to real path if path is different from info.path */
         private realpath: Path | undefined;
 
+        /*@internal*/
+        cacheSourceFile: DocumentRegistrySourceFileCache;
+
         constructor(
             private readonly host: ServerHost,
             readonly fileName: NormalizedPath,