Reland "[DFSan] Handle fast16labels for all API functions."

Support fast16labels in `dfsan_has_label`, and print an error for all
other API functions.  For `dfsan_dump_labels` we return silently rather
than crashing since it is also called from the atexit handler where it
is undefined behavior to call exit() again.

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D84215