swift234

Give SILFunctionTypes parallel generic signature and interface types.

For now, derive the generic signature from the contextual generic parameter list, so we can incrementally move producers and consumers of SILFunctionTypes to the new model independently. We derive the generic signature, but we can't yet derive the interface parameter and result types in all cases due to bugs in how we lower nested generic SILFunctionTypes. NFC yet.

Swift SVN r11722