swift234

[ClangImporter] Always treat dispatch_block_t as @convention(block).

Some GCD APIs rely on the pointer identity of blocks, so avoid bridging
when possible. The easiest way to do this was to use our existing rules
for special-casing typedefs.

The summary text for dispatch_block_t comes from the actual GCD headers.

rdar://problem/22432170

Swift SVN r31634