swift234

Reflection: Preliminary implementation of struct and tuple type lowering

Now that we can parse and substitute typerefs, and look up field
types, we finally have enough infrastructure in place to do some
basic layout of struct and tuple types from within the Reflection
library.

To facilitate testing, swift-reflection-dump now accepts multiple
-binary-filename flags, allowing types defined in the standard
library to be looked up.

More detailed end-to-end tests will come once I finish the
typeref-to-metadata builder.