[Clang-Tidy: modernize] Fix for modernize-redundant-void-arg: complains about variable cast to void

Summary:
Hello, i would like to suggest a fix for one of the checks in clang-tidy.The bug was reported in https://bugs.llvm.org/show_bug.cgi?id=32575 where you can find more information.

For example:
```
template <typename T0>
struct S {
  template <typename T>
  void g() const {
    int a;
    (void)a;
  }
};

void f() {
  S<int>().g<int>();
}
```


this piece of code should not trigger any warning by the check modernize-redundant-void-arg but when we execute the following command


```
clang_tidy -checks=-*,modernize-redundant-void-arg test.cpp -- -std=c++11
```

we obtain the following warning:

/Users/eco419/Desktop/clang-tidy.project/void-redundand_2/test.cpp:6:6: warning: redundant void argument list in function declaration [modernize-redundant-void-arg]
    (void)a;
     ^~~~

Reviewers: aaron.ballman, hokein, alexfh, JonasToth

Reviewed By: aaron.ballman, JonasToth

Subscribers: JonasToth, lebedev.ri, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D52135

llvm-svn: 342388