[DFSan] Support fast16labels mode in dfsan_union.

While the instrumentation never calls dfsan_union in fast16labels mode,
the custom wrappers do.  We detect fast16labels mode by checking whether
any labels have been created.  If not, we must be using fast16labels
mode.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D86012