[MSAN] Add fiber switching APIs

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.

Fixes google/sanitizers#1232

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D86471