[SyntaxTree] Expand support for `NestedNameSpecifier`

Summary:
We want NestedNameSpecifier syntax nodes to be generally supported, not
only for `DeclRefExpr` and `DependentScopedDeclRefExpr`.

To achieve this we:
* Use the `RecursiveASTVisitor`'s API to traverse
`NestedNameSpecifierLoc`s and automatically create its syntax nodes
* Add links from the `NestedNameSpecifierLoc`s to their syntax nodes.

In this way, from any semantic construct that has a `NestedNameSpecifier`,
we implicitly generate its syntax node via RAV and we can easily access
this syntax node via the links we added.