Only scan global sections containing data in LSan on darwin

Summary:
__DATA segments on Darwin contain a large number of separate sections,
most of which cannot actually contain pointers, and contain const values or
objc metadata. Only scanning sections which can contain pointers greatly improves
performance.

On a medium-sized (~4000 files) internal project, I saw a speedup of about 50%
in standalone LSan's execution time (50% improvement in the time spent running
LSan, not the total program time).

Reviewers: kcc, kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35432

llvm-svn: 308231