[analyzer] Fix crash on modeling of pointer arithmetic

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314141 91177308-0d34-0410-b5e6-96231b3b80d8