cmArgumentParser: Fix -Wcomma warning

Clang `-Wcomma` warns:

```
Source/cmArgumentParser.cxx:58:42: warning: possible misuse of comma operator
  this->CurrentList = (val.emplace_back(), &val.back());
                                         ^
```

This was introduced by commit 4359fe133b (Introduce cmArgumentParser,
2019-03-23).  Suppress it with the suggested cast.

Source/cmArgumentParser.cxx

@@ -55,7 +55,7 @@ void Instance::Bind(StringList& val)
 void Instance::Bind(MultiStringList& val)
 {
   this->CurrentString = nullptr;
-  this->CurrentList = (val.emplace_back(), &val.back());
+  this->CurrentList = (static_cast<void>(val.emplace_back()), &val.back());
   this->ExpectValue = false;
 }