[clang-scan-deps] We need to support C++20 Named Modules

In C++20 Named Modules, we introduced `ImportDecl` to describe the dependencies between modules. But this haven't been handled by clang-scan-deps.

For example,

```C++
// A.cppm
export module A;
...

// B.cpp
import A;
```

Currently we can't find that A.cppm is a dependence of B.cpp.