[asan] allow asan to use SizeClassAllocator32 on a 64-bit platform (if SANITIZER_CAN_USE_ALLOCATOR64=0). No functionality change by default is intended

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@198044 91177308-0d34-0410-b5e6-96231b3b80d8