[include-cleaner] Ignore the ParmVarDecl itself in WalkAST.cpp

This fixes a false positive where a ParamVarDecl happend to be the
same name of some C standard symbol and has a global namespace.

```
using A = int(int time); // we suggest <ctime> for the `int time`.
```

Differential Revision: https://reviews.llvm.org/D153330