Problem:
Visual Studio hangs while opening a solution with an XML file (such as a vsixmanifest) with the message that it's "Waiting on parse to complete".
Solution:
This is reported to be a known issue with Visual Studio 2017 (https://developercommunity.visualstudio.com/content/problem/44851/vs2017-hangs-on-waiting-for-parse-to-complete.html).
The short term work around is to go into your solutions directory and delete the .suo files. After doing so Visual Studio will open the solution fine.