Add lsan interceptors for libdispatch functions on darwin

Summary:
This is required for standalone LSan to work with libdispatch worker threads,
and is a slimmed down version of the functionality provided for ASan
in asan_mac.cc.

Reviewers: alekseyshl, kubamracek, glider, kcc

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D34247

llvm-svn: 305695