swift234

[ClangImporter] dispatch_block_t may be nullable or nonnull.

r31634 mistakenly treated all dispatch_block_ts as nonnull, because we've
never had to deal with a mapped typedef that refers to a pointer type.

('SEL' will eventually have the same issue, but for now the Swift
'Selector' type just has nil as an inhabitant.)

rdar://problem/22843921

Swift SVN r32210