For now this only computes the allocated size of the memory pointed to by a
pointer, and offset a pointer from allocated pointer.
The actual checkLimits part will come later, after another round of review.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75657 91177308-0d34-0410-b5e6-96231b3b80d8