tsan: Introducing a function to flush the shadow memory from third-party libraries

As discussed with Dmitry (https://goo.gl/SA4izd), I would like to introduce a function to be called from a third-party library to flush the shadow memory.
In particular, we ran some experiments with our tool Archer (an OpenMP data race detector based on Tsan, https://github.com/PRUNER/archer) and flushing the memory at the end of an outer parallel region, slightly increase the runtime overhead, but reduce the memory overhead of about 30%. This feature would come very handy in case of very large OpenMP applications that may cause an "out of memory" exception when checked with Tsan.

Reviewed in: https://reviews.llvm.org/D28443
Author: Simone Atzeni (simoatze)

llvm-svn: 291346