[dfsan] Introduce dfsan_flush().

Summary:
dfsan_flush() allows to restart tain tracking from scratch in the same process.
The primary purpose right now is to allow more efficient data flow tracing
for DFT fuzzing: https://github.com/google/oss-fuzz/issues/1632

Reviewers: pcc

Reviewed By: pcc

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63037

llvm-svn: 363321