[analyzer] Avoid checking addrspace pointers in cstring checker

This change fixes an assert that occurs in the SMT layer when refuting a
finding that uses pointers of two different sizes. This was found in a
downstream build that supports two different pointer sizes, The CString
Checker was attempting to compute an overlap for the 'to' and 'from'
pointers, where the pointers were of different sizes.

In the downstream case where this was found, a specialized memcpy
routine patterned after memcpy_special is used. The analyzer core hits
on this builtin because it matches the 'memcpy' portion of that builtin.
This cannot be duplicated in the upstream test since there are no
specialized builtins that match that pattern, but the case does
reproduce in the accompanying LIT test case. The amdgcn target was used
for this reproducer. See the documentation for AMDGPU address spaces here
https://llvm.org/docs/AMDGPUUsage.html#address-spaces.

The assert seen is:

`*Solver->getSort(LHS) == *Solver->getSort(RHS) && "AST's must have the same sort!"'

Ack to steakhal for reviewing the fix, and creating the test case.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D118050