AST: Be more careful about calls to ErrorType::get()

It's nice to be able to set a breakpoint on this method when
debugging the type checker, but recently we started unconditionally
calling it to get the circularity sentinel before kicking off a
request.

Instead, let's only call it when we're in the failure path.

Author: slavapestov

lib/AST/Decl.cpp

@@ -1256,9 +1256,11 @@ AccessLevel ExtensionDecl::getMaxAccessLevel() const {
 
 Type ExtensionDecl::getExtendedType() const {
   ASTContext &ctx = getASTContext();
-  return evaluateOrDefault(ctx.evaluator,
-    ExtendedTypeRequest{const_cast<ExtensionDecl *>(this)},
-    ErrorType::get(ctx));
+  if (auto type = evaluateOrDefault(ctx.evaluator,
+          ExtendedTypeRequest{const_cast<ExtensionDecl *>(this)},
+          Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 /// Clone the given generic parameters in the given list. We don't need any
@@ -2881,19 +2883,21 @@ bool ValueDecl::isRecursiveValidation() const {
 }
 
 Type ValueDecl::getInterfaceType() const {
+  auto &ctx = getASTContext();
+
   // Our clients that don't register the lazy resolver are relying on the
   // fact that they can't pull an interface type out to avoid doing work.
   // This is a necessary evil until we can wean them off.
-  if (!getASTContext().getLazyResolver()) {
+  if (!ctx.getLazyResolver()) {
     return TypeAndAccess.getPointer();
   }
 
-  if (auto Ty =
-          evaluateOrDefault(getASTContext().evaluator,
+  if (auto type =
+          evaluateOrDefault(ctx.evaluator,
                             InterfaceTypeRequest{const_cast<ValueDecl *>(this)},
-                            ErrorType::get(getASTContext())))
-    return Ty;
-  return ErrorType::get(getASTContext());
+                            Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 void ValueDecl::setInterfaceType(Type type) {
@@ -3695,9 +3699,11 @@ SourceRange TypeAliasDecl::getSourceRange() const {
 
 Type TypeAliasDecl::getUnderlyingType() const {
   auto &ctx = getASTContext();
-  return evaluateOrDefault(ctx.evaluator,
+  if (auto type = evaluateOrDefault(ctx.evaluator,
            UnderlyingTypeRequest{const_cast<TypeAliasDecl *>(this)},
-           ErrorType::get(ctx));
+           Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 void TypeAliasDecl::setUnderlyingType(Type underlying) {
@@ -3728,10 +3734,12 @@ UnboundGenericType *TypeAliasDecl::getUnboundGenericType() const {
 
 Type TypeAliasDecl::getStructuralType() const {
   auto &ctx = getASTContext();
-  return evaluateOrDefault(
+  if (auto type = evaluateOrDefault(
       ctx.evaluator,
       StructuralTypeRequest{const_cast<TypeAliasDecl *>(this)},
-      ErrorType::get(ctx));
+      Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 Type AbstractTypeParamDecl::getSuperclass() const {
@@ -6313,9 +6321,11 @@ void SubscriptDecl::setIndices(ParameterList *p) {
 Type SubscriptDecl::getElementInterfaceType() const {
   auto &ctx = getASTContext();
   auto mutableThis = const_cast<SubscriptDecl *>(this);
-  return evaluateOrDefault(ctx.evaluator,
+  if (auto type = evaluateOrDefault(ctx.evaluator,
                            ResultTypeRequest{mutableThis},
-                           ErrorType::get(ctx));
+                           Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 ObjCSubscriptKind SubscriptDecl::getObjCSubscriptKind() const {
@@ -6943,9 +6953,11 @@ StaticSpellingKind FuncDecl::getCorrectStaticSpelling() const {
 Type FuncDecl::getResultInterfaceType() const {
   auto &ctx = getASTContext();
   auto mutableThis = const_cast<FuncDecl *>(this);
-  return evaluateOrDefault(ctx.evaluator,
+  if (auto type = evaluateOrDefault(ctx.evaluator,
                            ResultTypeRequest{mutableThis},
-                           ErrorType::get(ctx));
+                           Type()))
+    return type;
+  return ErrorType::get(ctx);
 }
 
 bool FuncDecl::isUnaryOperator() const {