swift234

[ClangImporter] Don't put forward-declared structs in the lookup table.

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