"Force Include" config UI should not report validation errors. Whether a forced include can be resolved is unknown until compiler is queried.

[michelleangela]

Bug type: Language Service

Describe the bug

• OS and Version: Windows
• C/C++ Extension Version: 1.5.1

Steps to reproduce

1. Enter an invalid path in "Force include" property in IntelliSense config UI. There should be error saying cannot find path.
2. Open the c_cpp_properties.json file
3. Observe there is not error in json file for invalid path
   

Expected behavior
UI and json file should show the same errors if the current selected active configuration is the one also show on config UI

[Colengms]

If not a full path, a forced include may refer to a partial path (or filename) that will be resolved against all include paths (including system include paths, which are not available at the time of validation). So, I think the more correct behavior may be to not report validation issues here at all.

[bobbrow]

If we can validate absolute paths, let's do that. We can ignore validation errors for relative paths.