[gfortran] Fix NULL reference types.

[Nathan Sidwell]

Richard Henderson wrote:

> Now, from an optimization standpoint, sure he can go ahead and
> use a pointer type and everything works.  But from a debugging
> standpoint, we've just messed things up.  And I'm not sure how
> best to go about rectifying that.
aah, I see.  Perhaps we need a lang_hook 'ref_type_can_be_null', or
a flag on the type itself TYPE_REFERENCE_MAY_BE_NULL.

nathan

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