[asan] Add an interceptor for strtok

Summary:
This change addresses https://github.com/google/sanitizers/issues/766. I
tested the change with make check-asan and the newly added test case.

Reviewers: ygribov, kcc, alekseyshl

Subscribers: kubamracek, llvm-commits

Patch by mrigger

Differential Revision: https://reviews.llvm.org/D30384

llvm-svn: 298650