"Configure your IntelliSense settings" displays on startup with file already open, if using a custom config provider

Repro steps:

• Use a workspace folder configured using a custom configuration provider.
• Leave a .cpp file open when the workspace folder was last used.
• Open that workspace folder.

Seeing:

Note that there are no 'problems' here to resolve.

Expected: No prompt to fix my missing headers when there are none missing.

This is likely happening for everyone who use custom configuration providers (including those who are automatically configured to use one), any time they open a workspace folder that already has a .cpp file open.

When cpptools starts up, there is a brief period of time in which the custom configuration would not yet be registered. Rather than wait for it to register, a TU is started (for the already open file in the editor) with the base configuration, which likely will not be a proper configuration for the file. Moments later, as the custom configuration provider registers, the TU is recreated, and any transient errors are cleared out. Yet, this dialog is still displayed.

One possible solution would be to delay this dialog until a certain amount of time after start-up, and not display it if whatever issue that triggered it has already been resolved.

Ideally, we could remove this prompt whenever we see that there is no longer any such issue to address. If it's not possible to programmatically dismiss the dialog, perhaps we should open a feature request for that against VS Code.

Start with startup handling for an already-open .cpp file, custom configuration provider registration, and the "Configure your IntelliSense settings" prompt. Reproduce the sequence in the issue, then verify that the prompt is absent after the custom configuration registers when no missing-header problem remains.