[dfsan] Expose dfsan_get_track_origins to get origin tracking status

This allows application code checks if origin tracking is on before
printing out traces.

-dfsan-track-origins can be 0,1,2.
The current code only distinguishes 1 and 2 in compile time, but not at runtime.
Made runtime distinguish 1 and 2 too.

Reviewed By: browneee

Differential Revision: https://reviews.llvm.org/D105128