[tsan] Provide API for libraries for race detection on custom objects

This patch allows a non-instrumented library to call into TSan runtime, and tell us about "readonly" and "modifying" accesses to an arbitrary "object" and provide the caller and tag (type of object).  This allows TSan to detect violations of API threading contracts where "read-only" methods can be called simulatenously from multiple threads, while modifying methods must be exclusive.

Differential Revision: https://reviews.llvm.org/D28836

llvm-svn: 293885