Enable C/C++: Log Diagnostics to be used without an active C/C++ file

[sean-mcmanus]

That would be helpful, since it shows relevant info regardless of if a C/C++ file is active or not (e.g. if a settings or c_cpp_properties.json is active instead).

I think a long time ago, it only showed info on the currently active TU so that's why we had the active file requirement (some code related to the "active" file might still need to be changed though).