Add a notification to the status bar when selecting a compiler will improve IntelliSense

[bobbrow]

Feature Request

There are many ways to configure IntelliSense, and to be the most effective we are looking to show this to a specific subgroup of users who will get the most help out of being redirected to the compiler quickpick. These users are:

• Users who have not configured IntelliSense using a compiler, custom configuration provider, or compile_commands.json
• Users who have configured a custom configuration provider or compile_commands, however they are falling back on the base configuration instead
  For these groups, we will add the item (design below) to the status bar. If we determine that users do not meet the requirements above anymore, the status bar icon will be removed.

Clicking on this will pull up the quickpick to select a compiler (the same effect as the Select Default Compiler command in the command pallete). Once a user has configured a compiler for IntelliSense or made a selection in the quickpick (such as selecting a compiler or choosing “”), this item will disappear from the status bar.

[bobbrow]

This issue replaces #10527