[tsan] Pass correct interposed function prefix to report function

Summary:
On Darwin, interposed functions are prefixed with "wrap_". On Linux,
they are prefixed with "__interceptor_".

Reviewers: dvyukov, samsonov, glider, kcc, kubabrecka

Subscribers: zaks.anna, llvm-commits

Differential Revision: http://reviews.llvm.org/D14512

llvm-svn: 252695