Clean up the sentinel-attribute checking code a lot. Document

Clean up the sentinel-attribute checking code a lot.  Document
what 'nullPos' is supposed to mean, at least at this one site.
Use closed forms for the arithmetic.  Rip out some clever but
ultimately pointless code that was trying to use 0 or 0L depending
the size of a pointer vs. the size of int;  first, it didn't work
on LLP64 systems, and second, the sentinel checking code requires
a pointer-typed value anyway, so this fixit would not have actually
removed the warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139361 91177308-0d34-0410-b5e6-96231b3b80d8