Revert to C-style callbacks for iteration over allocator chunks.

Also clean up LSan code, fix some comments and replace void* with uptr
to bring down the number of reinterpret_casts.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@184700 91177308-0d34-0410-b5e6-96231b3b80d8