makeNull needs deprecation in favor of makeNullWithType

[vabridgers]

A recent issue uncovered with address spaces and the CString Checker exposed the need to deprecate makeNull in SValBuilder in favor of a method to make a null pointer that "right sized" per the prototype pointer's address space. See https://reviews.llvm.org/D118050 for details of the problem.

This will involve the following steps:

1. Replace instances of makeNull with makeNullWithType
2. Replace instances of makeIntValWithPtrWidth with a more appropriate method to comprehend expected pointer size
3. Replace instances of getIntWithPtrWidth with a more appropriate method to comprehend expected pointer size

[steakhal]

+1

[steakhal]

Only the first patch has landed out of three if I'm correct.

[steakhal]

The rest of the patchset has landed.