[analyzer] Report bugs when freeing memory with offset pointer

The malloc checker will now catch the case when a previously malloc'ed
region is freed, but the pointer passed to free does not point to the
start of the allocated memory. For example:

int *p1 = malloc(sizeof(int));
p1++;
free(p1); // warn

From the "memory.LeakPtrValChanged enhancement to unix.Malloc" entry
in the list of potential checkers.

A patch by Branden Archer!

llvm-svn: 174678