Fix bug: 'import' functions cannot get a body at the same time

[fernewelten]

Fixes #2617

The compiler didn't catch functions that are declared import and at the same time get a body. This is an error.

Example:

import int Func()   // ←'import' keyword
{    // ← function has a body
    return 0;
}

Add a Googletest that checks that this is now illegal.