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.

tools/swift-ide-test/ModuleAPIDiff.cpp

@@ -754,6 +754,7 @@ public:
     }
     for (auto &Req : GS->getRequirements()) {
       switch (Req.getKind()) {
+      case RequirementKind::Superclass:
       case RequirementKind::Conformance:
         ResultGS.ConformanceRequirements.emplace_back(
             sma::ConformanceRequirement{