[analyzer] Avoid out-of-order node traversal on void return (#117863)

The motivating example: https://compiler-explorer.com/z/WjsxYfs43
```C++
#include <stdlib.h>
void inf_loop_break_callee() {
  void* data = malloc(10);
  while (1) {
    (void)data; // line 3
    break; // -> execution continues on line 3 ?!!
  }
}
```

To correct the flow steps in this example (see the fixed version in the
added test case) I changed two things in the engine:
- Make `processCallExit` create a new StmtPoint only for return
  statements. If the last non-jump statement is not a return statement,
  e.g. `(void)data;`, it is no longer inserted in the exploded graph after
  the function exit.
- Skip the purge program points. In the example above, purge
  points are still inserted after the `break;` executes. Now, when the bug
  reporter is looking for the next statement executed after the function
  execution is finished, it will ignore the purge program points, so it
  won't confusingly pick the `(void)data;` statement.

CPP-5778