Remove SimpleRequest::breakCycle

This patch removes the need for Request objects to provide a default
cycle-breaking value, instead opting to return llvm::Expected so clients
must handle a cycle failure explicitly.

Currently, all clients do the 'default' behavior, but this opens the
possibility for future requests to handle failures explicitly.

lib/AST/NameLookup.cpp

@@ -2168,8 +2168,9 @@ resolveTypeDeclsToNominal(Evaluator &evaluator,
       if (!typealiases.insert(typealias).second)
         continue;
 
-      auto underlyingTypeReferences
-        = evaluator(UnderlyingTypeDeclsReferencedRequest{typealias});
+      auto underlyingTypeReferences = evaluateOrDefault(evaluator,
+        UnderlyingTypeDeclsReferencedRequest{typealias}, {});
+
       auto underlyingNominalReferences
         = resolveTypeDeclsToNominal(evaluator, ctx, underlyingTypeReferences,
                                     modulesFound, anyObject, typealiases);
@@ -2443,12 +2444,13 @@ DirectlyReferencedTypeDecls UnderlyingTypeDeclsReferencedRequest::evaluate(
 }
 
 /// Evaluate a superclass declaration request.
-ClassDecl *SuperclassDeclRequest::evaluate(Evaluator &evaluator,
-                                           NominalTypeDecl *subject) const {
+llvm::Expected<ClassDecl *>
+SuperclassDeclRequest::evaluate(Evaluator &evaluator,
+                                NominalTypeDecl *subject) const {
   for (unsigned i : indices(subject->getInherited())) {
     // Find the inherited declarations referenced at this position.
-    auto inheritedTypes =
-      evaluator(InheritedDeclsReferencedRequest{subject, i});
+    auto inheritedTypes = evaluateOrDefault(evaluator,
+      InheritedDeclsReferencedRequest{subject, i}, {});
 
     // Resolve those type declarations to nominal type declarations.
     SmallVector<ModuleDecl *, 2> modulesFound;
@@ -2467,8 +2469,9 @@ ClassDecl *SuperclassDeclRequest::evaluate(Evaluator &evaluator,
   return nullptr;
 }
 
-NominalTypeDecl *ExtendedNominalRequest::evaluate(Evaluator &evaluator,
-                                                  ExtensionDecl *ext) const {
+llvm::Expected<NominalTypeDecl *>
+ExtendedNominalRequest::evaluate(Evaluator &evaluator,
+                                 ExtensionDecl *ext) const {
   DirectlyReferencedTypeDecls referenced;
   ASTContext &ctx = ext->getASTContext();
 
@@ -2504,7 +2507,8 @@ void swift::getDirectlyInheritedNominalTypeDecls(
                              : extDecl->getASTContext();
 
   // Find inherited declarations.
-  auto referenced = ctx.evaluator(InheritedDeclsReferencedRequest{decl, i});
+  auto referenced = evaluateOrDefault(ctx.evaluator,
+    InheritedDeclsReferencedRequest{decl, i}, {});
 
   // Resolve those type declarations to nominal type declarations.
   SmallVector<ModuleDecl *, 2> modulesFound;