[analyzer] Track null or undef values through pointer arithmetic.

Pointer arithmetic on null or undefined pointers results in null or undefined
pointers. This is obvious for undefined pointers; for null pointers it follows
from our incorrect-but-somehow-working approach that declares that 0 (Loc)
doesn't necessarily represent a pointer of numeric address value 0, but instead
it represents any pointer that will cause a valid "null pointer dereference"
issue when dereferenced.

For now we've been seeing through pointer arithmetic at the original dereference
expression, i.e. in bugreporter::getDerefExpr(), but not during further
investigation of the value's origins in bugreporter::trackNullOrUndefValue().
The patch fixes it.

Differential Revision: https://reviews.llvm.org/D45071


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328896 91177308-0d34-0410-b5e6-96231b3b80d8