Introduce a new representation of polymorphic function types.

Introduces a new kind of function type, GenericFunctionType, that
represents a polymorphic function type with all of its generic
parameters and requirements stored in a more readily canonicalizable
form. It is meant to eventually replace PolymorphicFunctionType, but
for now we build it up in parallel so we can switch over to it
pieacemeal.

Note: this representation is built and then thrown away. We'll start
recording it soon.


Swift SVN r8881

lib/Serialization/Serialization.cpp

@@ -1493,6 +1493,10 @@ void Serializer::writeType(Type ty) {
     break;
   }
 
+  case TypeKind::GenericFunction: {
+    llvm_unreachable("Cannot serialize generic function types yet");
+  }
+      
   case TypeKind::GenericTypeParam: {
     auto genericParam = cast<GenericTypeParamType>(ty.getPointer());
     unsigned abbrCode = DeclTypeAbbrCodes[GenericTypeParamTypeLayout::Code];