This is a request for API access to the original encoding of a file. i.e.:

(If this is already possible, please let me know. I couldn't find it.)

If I understand correctly, VS Code represents the content of open files to extensions as UTF-8. In the C/C++ Extension, there are scenarios in which IntelliSense should reflect how the compiler would interpret the file, which may depend on the files actual encoding. i.e. MSVC and string literal contents.