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
VS2019 still has this issue. Delete .vs\SolutionName\v17\.suo
If deleting .suo still does solve your problem, then delete also all bin and obj directories in every project in the solution.