[analyzer] Fix crash on modeling of pointer arithmetic

This patch fixes analyzer's crash on the newly added test case 
(see also https://bugs.llvm.org/show_bug.cgi?id=34374).
Pointers subtraction appears to be modeled incorrectly 
in the following example:
  char* p;
  auto n = p - reinterpret_cast<char*>((unsigned long)1);
In this case the analyzer (built without this patch) 
tries to create a symbolic value for the difference 
treating reinterpret_cast<char*>((unsigned long)1) 
as an integer, that is not correct.

Differential revision: https://reviews.llvm.org/D38214

Test plan: make check-all

llvm-svn: 314141