swift234

Clang importer: Add a zeroing default initializer to imported structs.

If an imported C struct has no __nonnull pointer fields, then we can give a default initializer that zeroes all of its fields. This becomes a requirement when working with partially-imported types like NSDecimal. NSDecimal has bitfields Swift can't see yet, so it's impossible to DI, but the Foundation functions that work with NSDecimal all emit their result by out parameter, and without access to its fields it is impossible to initialize an NSDecimal for use with one of these functions. Implement the initializer using a builtin that gets lowered by IRGen; this is also made necessary by the fact that Swift has only a partial view of the struct, so we can't form a complete zero initializer until we have the definitive type layout from Clang.

Swift SVN r23727