[clang-doc] Add typedef/using information.

Read typedef and "using" type alias declarations and serialize into the internal structures. Emit this information in the YAML output. The HTML and MD generators are unchanged.

Separate out the logic to create the parent namespace or record object and insert the newly created child into it. This logic was previously duplicated for every "info" type and is now shared.

To help this, a struct containing the child vectors was separated out so children can be added generically and without having too many templates.

A small change was made to populateParentNamespaces() to allow using types that aren't themselves DeclContexts (typedefs are the first example of this).

Reviewed By: paulkirth, haowei

Differential Revision: https://reviews.llvm.org/D134371