swift234

[serialization] Load the decl and type offset arrays from the module.

...but don't do anything with them yet. This does check that they're being
correctly serialized, though.

This introduces a new ADT, PointerIntUnion, which like PointerUnion is an
efficient variant type using the lowest bit of data as a discriminator.
By default, the union can store any pointer-bits-minus-one-sized integer,
but both the integer type and the underlying storage type can be
customized.

Swift SVN r5321