Expose --fixed-format-cache if compiled (#19815)

Author: ilevkivskyi

mypy/main.py

@@ -1064,7 +1064,13 @@ def add_invertible_flag(
         help="Include fine-grained dependency information in the cache for the mypy daemon",
     )
     incremental_group.add_argument(
-        "--fixed-format-cache", action="store_true", help=argparse.SUPPRESS
+        "--fixed-format-cache",
+        action="store_true",
+        help=(
+            "Use experimental fast and compact fixed format cache"
+            if compilation_status == "yes"
+            else argparse.SUPPRESS
+        ),
     )
     incremental_group.add_argument(
         "--skip-version-check",