[Allocator] Lift the slab size and size threshold into template
parameters rather than runtime parameters.

There is only one user of these parameters and they are compile time for
that user. Making these compile time seems to better reflect their
intended usage as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205143 91177308-0d34-0410-b5e6-96231b3b80d8