[clang-tidy] Fix a false positive in modernize-use-nullptr.

Summary:
The FP happens when a casting nullptr expression is used within a NULL-default-arguemnt cxx constructor.

Before the fix, the check will give a warning on nullptr when running
with the test case, which should not happen:

```
G(g(static_cast<char*>(nullptr)));
    ^~~~~~~~~~~
    nullptr
```

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, xazax.hun

Differential Revision: https://reviews.llvm.org/D34524

llvm-svn: 306091