Create iterator for ASTDeclInterface.

[anthro-poid]

          We will need something like `filed_range = llvm::iterator_range< specific_decl_iterator< vast::ASTFieldDeclInterface > >`.

This might be more complex so create it an issue and lets resolve it later. Keep vector for now.

Originally posted by @xlauko in #661 (comment)

[anthro-poid]

Some methods in the ASTDeclInterface interfaces need to return iterable objects. Currently, std::vector is used as a temporary solution, but it is not an optimal solution. A suitable solution is to create a special iterator for these interfaces.