Have PreXmlDoc/XmlDoc track names and ranges for param/paramref children

**Is your feature request related to a problem? Please describe.**

I wanted to make xmldocs participate in rename in FsAutoComplete, so that when a user renamed a constructor parameter, or function binding parameter, etc. the matching xmlDoc on that element, as well as any `paramRef`s referring to that parameter, would be updated as well. This turned out to be very difficult.

**Describe the solution you'd like**

I'd like the PreXmlDoc/XmlDoc structures to track the names (including source-file-ranges of the actual name string) of the parsed Xml Doc Comments for structures that can have parameters. This includes, but may not be limited to

* implicit and explicit constructors
* DU case type constructors
* let bindings that describe functions

**Describe alternatives you've considered**

FSAC starts doing _a lot_ of re-parsing of XmlDocs :)