[SourceKitd] Avoid printing parent type directly on extended type. rdar://37965902 (#14915)

When printing parent type directly, we may print generic arguments and sugared
dictionary type.

Author: nkcsgexi

lib/AST/ASTPrinter.cpp

@@ -1751,15 +1751,15 @@ static void printExtendedTypeName(Type ExtendedType, ASTPrinter &Printer,
                                   PrintOptions Options) {
   auto Nominal = ExtendedType->getAnyNominal();
   assert(Nominal && "extension of non-nominal type");
-  if (auto ct = ExtendedType->getAs<ClassType>()) {
-    if (auto ParentType = ct->getParent()) {
-      ParentType.print(Printer, Options);
-      Printer << ".";
-    }
-  }
-  if (auto st = ExtendedType->getAs<StructType>()) {
-    if (auto ParentType = st->getParent()) {
-      ParentType.print(Printer, Options);
+  if (auto nt = ExtendedType->getAs<NominalType>()) {
+    if (auto ParentType = nt->getParent()) {
+      if (auto *PD = ParentType->getNominalOrBoundGenericNominal()) {
+        // Avoid using the parent type directly because it can be bound
+        // generic type and sugared.
+        PD->getDeclaredType().print(Printer, Options);
+      } else {
+        ParentType.print(Printer, Options);
+      }
       Printer << ".";
     }
   }

test/SourceKit/DocSupport/Inputs/cake1.swift

@@ -25,3 +25,7 @@ public protocol P3 {
 public extension P2 where Self : P3 {
   func fooConstraint() {}
 }
+
+public extension Dictionary.Keys {
+  public func foo() {}
+}