Improvement: Re-generate diagnostics when change active text editor

Looks like diagnostics are only generated when text document is changed. However, the following pattern is very common:
1. Change file A and save. You get diagnostics information for file A.
2. Switch to file B, for example delete a class that used in A, and then save.
3. Switch to file A. **Diagnostics such as errors and warnings should be regenerated**

I am using vscode 1.49.2 and OmniSharp 1.23.2.