Add another visibility tag (#26644)

* Update visibility tags

Add some tags to the bazel build that we can leverage internally to help
control visibility in the Google build system.

* review feedback

* Add needed visibility tag

BUILD

@@ -2796,6 +2796,7 @@ grpc_cc_library(
     ],
     language = "c++",
     public_hdrs = GRPCXX_PUBLIC_HDRS,
+    visibility = ["@grpc:alt_grpc++_base_unsecure_legacy"],
     deps = [
         "gpr_base",
         "grpc++_codegen_base",

bazel/grpc_build_system.bzl

@@ -76,6 +76,7 @@ def _update_visibility(visibility):
         "alt_gpr_base_legacy": PRIVATE,
         "alt_grpc++_base_legacy": PRIVATE,
         "alt_grpc_base_legacy": PRIVATE,
+        "alt_grpc++_base_unsecure_legacy": PRIVATE,
         "alts_frame_protector": PRIVATE,
         "client_channel": PRIVATE,
         "debug_location": PRIVATE,