If the declaration is associated to an AST node return it, otherwise it return empty.
Declaration based on source code have an AST node associated while others don't. Example
of other declarations are declarations coming from reflection or JARs.
You may wonder how this method is different from the various getWrappedNode.
The difference is that toAst is present in all Resolved* declarations (such as
ResolvedAnnotationDeclaration), while getWrappedNode is present
only on the subclasses of the Resolved* declarations that derive from JP AST nodes (such as
JavaParserClassDeclaration). Therefore one
which has a Resolved* declaration need to do a downcast before being able to use getWrappedNode.
Now, this means that toAst could potentially replace getWrappedNode (but not the other way around!).
However toAst return an Optional, which is less convenient than getting the direct node. Also,
toAst sometimes have to return a more generic node. This is the case for subclasses of
ResolvedClassDeclaration. In those cases toAst return a Node. Why? Because both anonymous
class declarations and standard class declarations are subclasses of that. In one case the
underlying AST node is an ObjectCreationExpr, while in the other case it is ClassOrInterfaceDeclaration.
In these cases getWrappedNode is particularly nice because it returns the right type of AST node,
not just a Node.