[sanitizer] allocator: introduce kUseSeparateSizeClassForBatch (false by...

[sanitizer] allocator: introduce kUseSeparateSizeClassForBatch (false by default). When true, it will cause all TransferBatches to be allocated on a separate dedicated size class, which improves security and may potentially simplify memory reclamation. However in the current state this may cause up to 3% extra memory usage. Subsequent changes should bring this overhead down

llvm-svn: 276377