1) Clones llvm::BitVector, and makes it Allocator aware (using the
CfgLocalAllocator<>)

2) Uses mallopt to set the malloc granularity. The default granularity
is too small, which forces too many mmap calls.

BUG=
R=sehr@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1738683003 .