Silence qdoc warnings from std::filesystem

We change the declaration of the new APIs using std::filesystem::path for
qdoc runs, and need to forward declare it consistently for qdoc builds to
avoid a flood of clang warnings when building documentation.

Change-Id: Iddcf0ce7d6207b6cc5910790315ab21076bd6ce1
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/io/qfile.h

@@ -47,6 +47,13 @@
 
 #if QT_CONFIG(cxx17_filesystem)
 #include <filesystem>
+#elif defined(Q_CLANG_QDOC)
+namespace std {
+    namespace filesystem {
+        class path {
+        };
+    };
+};
 #endif
 
 #ifdef open