[lsan] Use real memset to clear memory in standalone LSan.

Performance improvement. Also, the allocator was using CompactSizeClassMap for
no good reason, so I switched it to DefaultSizeClassMap.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195570 91177308-0d34-0410-b5e6-96231b3b80d8