swift234

NSNumber bridging: use BOOL rather than _Bool when bridging Bools.

The Objective-C type encoding of Boolean values in NSNumber is that of
BOOL, which is either signed char or _Bool depending on the
platform. _SwiftTypePreservingNSNumber was using _Bool, which led to
inconsistencies when bridging vs. creating an NSNumber directly. Use
BOOL consistently.

Fixes rdar://problem/27894308.