Fix template instantiation of a non-dependent call to an inherited

constructor with default arguments.

We used to try to rebuild the call as a call to the faked-up inherited
constructor, which is only a placeholder and lacks (for example) default
arguments. Instead, build the call by reference to the original
constructor.

In passing, add a note to say where a call that recursively uses a
default argument from within itself occurs. This is usually pretty
obvious, but still at least somewhat useful, and would have saved
significant debugging time for this particular bug.