[clangd] Define out-of-line qualify function name

Summary:
When moving function definitions to a different context, the function
name might need a different spelling, for example in the header it might be:

```
namespace a {
  void foo() {}
}
```

And we might want to move it into a context which doesn't have `namespace a` as
a parent, then we must re-spell the function name, i.e:
```
void a::foo() {}
```

This patch implements a version of this which ignores using namespace
declarations in the source file.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70656