Properly set the name of `File::Constants`, which is the only name
with a namespace in `RDoc::KNOWN_CLASSES`, and fixes longstanding bug
that `File::Constants` becomes `File::File::Constants`.
When it is generated by `rb_file_const` in dir.c, `name` is set to the
qualified name as same as `full_name`, and generated in the normal way
in file.c later, already set `full_name` is cleared and `name` will be
constructed from the enclosing namespace and the `name`. It will
results in duplicated namespace, `File::File::Constants`.
https://github.com/ruby/rdoc/commit/3a8d6df562
ebcbca96fb