Distinguish conformance and superclass generic requirements.

As part of this, use a different enum for parsed generic requirements.

NFC except that I noticed that ASTWalker wasn't visiting the second
type in a conformance constraint; fixing this seems to have no effect
beyond producing better IDE annotations.

lib/Sema/CSGen.cpp

@@ -2912,10 +2912,13 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
       case RequirementKind::Conformance:
         createMemberConstraint(Req, ConstraintKind::ConformsTo);
         break;
+      case RequirementKind::Superclass:
+        createMemberConstraint(Req, ConstraintKind::Subtype);
+        break;
       case RequirementKind::SameType:
         createMemberConstraint(Req, ConstraintKind::Equal);
         break;
-      default:
+      case RequirementKind::WitnessMarker:
         break;
     }
   }