gen_stub: move getFileDocComments() into FileInfo

Reduce the number of global functions by moving it to static method
`FileInfo::getFileDocComments()`. Since it is only used by
`FileInfo::parseStubFile()`, also make it private.

Author: DanielEScherzer

build/gen_stub.php

@@ -4401,13 +4401,25 @@ protected function pName_FullyQualified(Name\FullyQualified $node): string {
         $stmts = $parser->parse($code);
         $nodeTraverser->traverse($stmts);
 
-        $fileTags = DocCommentTag::parseDocComments(getFileDocComments($stmts));
+        $fileTags = DocCommentTag::parseDocComments(self::getFileDocComments($stmts));
         $fileInfo = new FileInfo($fileTags);
 
         $fileInfo->handleStatements($stmts, $prettyPrinter);
         return $fileInfo;
     }
 
+    /** @return DocComment[] */
+    private static function getFileDocComments(array $stmts): array {
+        if (empty($stmts)) {
+            return [];
+        }
+
+        return array_filter(
+            $stmts[0]->getComments(),
+            static fn ($comment): bool => $comment instanceof DocComment
+        );
+    }
+
     private function handleStatements(array $stmts, PrettyPrinterAbstract $prettyPrinter): void {
         $conds = [];
         foreach ($stmts as $stmt) {
@@ -5143,18 +5155,6 @@ function parseClass(
     );
 }
 
-/** @return DocComment[] */
-function getFileDocComments(array $stmts): array {
-    if (empty($stmts)) {
-        return [];
-    }
-
-    return array_filter(
-        $stmts[0]->getComments(),
-        static fn ( $comment ): bool => $comment instanceof DocComment
-    );
-}
-
 /**
  * @template T
  * @param iterable<T> $infos