When trying to figure out errors from an import failure, the nullability
completeness warnings would clutter the output making it difficult to
identify the errors. Sprinkle the declaarations with
`_Null_unspecified` to maintain the current nullability semantics and
silence the warnings. NFC.
Otherwise, the lookup table for "CGColor" has two entries, because of
this:
typedef struct CGColor *CGColorRef;
and that interferes with our ability to import things as members of
"CGColor" (as opposed to "CGColorRef"), which affects the fix-its we
generate when you try to use the non-member form.
This isn't necessarily the best long-term solution (as noted in the
FIXME) but it is expedient and won't break any current users.
More rdar://problem/26347297
