BumpPtrAllocator: use uintptr_t when aligning addresses to avoid undefined behaviour

In theory, alignPtr() could push a pointer beyond the end of the current slab, making
comparisons with that pointer undefined behaviour. Use an integer type to avoid this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216973 91177308-0d34-0410-b5e6-96231b3b80d8