Add IntelliSense for C++20 modules importing

[sean-mcmanus]

See the previous Windows/cl.exe-only issue at #6290 (UPDATE: the issue was deleted, see #8256 (comment) ).

NOTE: C++20 modules importing works for cl.exe if you set /ifcSearchDir (and possibly other modules args if necessary); however the IntelliSense support is still in active development so there are lots of known issues/bugs still.

This is dependent on IntelliSense parser changes from VS (i.e. open file Linux solutions in VS don't have module importing support for IntelliSense either).

EDIT: example here: #6302 (comment)

[cjwijtmans]

Is there any way to get it somewhat working or does it not parse .ifc files at all?

[sean-mcmanus]

@cjwijtmans It may work with msvc mode with .ifc files for cl.exe, but it's not officialy supported yet. We process /headerUnit, /ifcSearchDir, /reference, and /stdIfcDir in compilerArgs.