[analyzer] Conjure a symbol to ensure we can identify pointer arithmetic

We need to identify the value of ptr as
ElementRegion (result of pointer arithmetic) in the following code.
However, before this commit '(2-x)' evaluated to Unknown value, and as
the result, 'p + (2-x)' evaluated to Unknown value as well.

int *p = malloc(sizeof(int));
ptr = p + (2-x);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156052 91177308-0d34-0410-b5e6-96231b3b80d8