Except GenericEnvironment.h, because you can't meaningfully use a
GenericEnvironment without its signature. Lots less depends on
GenericSignature.h now. NFC
Use a syntax that declares the layout's generic parameters and fields,
followed by the generic arguments to apply to the layout:
{ var Int, let String } // A concrete box layout with a mutable Int
// and immutable String field
<T, U> { var T, let U } <Int, String> // A generic box layout,
// applied to Int and String
// arguments
Trying to contain it in SIL doesn't really work if we want to be able to print or parse SIL box types, since the type parser and printer doesn't otherwise depend on the SIL module.
