Add functions exposed via the MSAN interface to enable MSAN within
binaries that perform manual stack switching (e.g. through using fibers
or coroutines).
This functionality is analogous to the fiber APIs available for ASAN and TSAN.
Fixesgoogle/sanitizers#1232
Reviewed By: vitalybuka
Differential Revision: https://reviews.llvm.org/D86471
Justin Cady