compiler: don't use predeclared location for implicit runtime import

    
    For the main package we add an implicit import of the runtime package,
    to ensure that it is initialized.  That import used the predeclared
    location, which caused various tests, notably Named_type::is_builtin,
    to treat these imported names as builtin.  Start using a real location,
    so that those tests do the right thing.
    
    By the way, this implicit import is a partial cause of golang/go#19773.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194337

From-SVN: r275540