swift234

SILGen: Three-parameter re-abstraction thunks

Right now, re-abstraction thunks are set up to convert values
as follows, where L is type lowering:

- OrigToSubst: L(origType, substType) -> L(substType)
- SubstToOrig: L(substType) -> L(origType, substType)

This assumes there's no AST-level type conversion, because
when we visit a type in contravariant position, we flip the
direction of the transform but we're still converting *to*
substType -- which will now equal to the type of the input,
not the type of the expected result!

This caused several problems:

- VTable thunk generation had a bunch of special logic to
  compute a substOverrideType, and wrap the thunk result
  in an optional, duplicating work done in the transform

- Witness thunk generation similarly had to handle the case
  of upcasting to a base class to call the witness, and
  casting the result of materializeForSet back to the right
  type for properties defined on the base.

  Now the materializeForSet cast sequence is a bit longer,
  we unpack the returned tuple and do a convert_function
  on the function, then pack it again -- before we would
  unchecked_ref_cast the tuple, which is technically
  incorrect since the tuple is not a ref, but IRGen didn't
  seem to care...

To handle the conversions correctly, we add a third AST type
parameter to a transform, named inputType. Now, transforms
perform these conversions:

- OrigToSubst: L(origType, inputType) -> L(substType)
- SubstToOrig: L(inputType) -> L(origType, substType)

When we flip the direction of the transform while visiting
types in contravariant position, we also swap substType with
inputType.

Note that this is similar to how bridging thunks work, for
the same reason -- bridging thunks convert between AST types.

This is mostly just a nice cleanup that fixes some obscure
corner cases for now, but this functionality will be used
in a subsequent patch.

Swift SVN r31486