[clang] removes check against integral-to-pointer conversion...

... unless it's a literal

D94640 was a bit too aggressive in its analysis, considering integers
representing valid addresses as invalid. This change rolls back some of
the check, so that only the most obvious case is still flagged.

Before:

```cpp
free((void*)1000);   // literal converted to `void*`: warning good
free((void*)an_int); // `int` object converted to `void*`: warning might
                     //  be a false positive
```

After

```cpp
free((void*)1000);   // literal converted to `void*`: warning good
free((void*)an_int); // doesn't warn
```

Differential Revision: https://reviews.llvm.org/D97512