Add an ArrayRecycler class.

This is similar to the existing Recycler allocator, but instead of
recycling individual objects from a BumpPtrAllocator, arrays of
different sizes can be allocated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171581 91177308-0d34-0410-b5e6-96231b3b80d8