I had thousands of intellisense errors and 0 build errors. After deleting .suo file and restarting VS intellisense errors are gone.
Suo file is located relatively to source in: .vs\SolutionName\v14\.suo
According to comment: Beware that *.suo is a hidden file.
Edit: According to comments, VS2017 has the same issue, so you can use similar solution: Delete .vs\SolutionName\v15\.suo