[analyzer] Anti-aliasing: different heap allocations do not alias

Add a concept of symbolic memory region belonging to heap memory space.
When comparing symbolic regions allocated on the heap, assume that they
do not alias. 

Use symbolic heap region to suppress a common false positive pattern in
the malloc checker, in code that relies on malloc not returning the
memory aliased to other malloc allocations, stack.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158136 91177308-0d34-0410-b5e6-96231b3b80d8