[asan] fix the alloctor code to not use opaque data structure, which was larger than needed. This was a leftover of the allocator1=>allocator2 migration; thanks Yuri Gribov for reminding

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@206280 91177308-0d34-0410-b5e6-96231b3b80d8