swift234

IRGen: Forward spare bits through aggregates.

When doing struct layout for fixed-layout structs or tuples, combine the spare bit masks of their elements to form the spare bit mask of the aggregate, treating padding between elements as spare bits as well.

For now, disable using these spare bits to form extra inhabitants for structs and tuples; we would need additional runtime work to expose these extra inhabitants for correct generic runtime behavior. This puts us in a weird situation where 'enum { case A(Struct), B, C }' spills a bit but 'enum { case A(Struct), B(Struct), C }' doesn't, but the work to make the former happen isn't immediately critical for String optimization.

Swift SVN r12165