[PrintAsObjC] Look through "compatibility" typealiases (#20027) (#20035)

These handle imported types that have been renamed in a /later/ Swift
version than the one being used; for consistency when deserializing
from a swiftmodule, the latest name is always used. This is important
because it might mean we can avoid importing the framework that a name
comes from; a forward declaration might be sufficient if it's an ObjC
class or protocol.

rdar://problem/45491607
(cherry picked from commit e34a6a1)

Author: jrose-apple

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -2014,10 +2014,12 @@ class ReferencedTypeFinder : public TypeVisitor<ReferencedTypeFinder> {
   }
 
   void visitNameAliasType(NameAliasType *aliasTy) {
-    if (aliasTy->getDecl()->hasClangNode())
+    if (aliasTy->getDecl()->hasClangNode() &&
+        !aliasTy->getDecl()->isCompatibilityAlias()) {
       Callback(*this, aliasTy->getDecl());
-    else
+    } else {
       visit(aliasTy->getSinglyDesugaredType());
+    }
   }
 
   void visitParenType(ParenType *parenTy) {

test/PrintAsObjC/versioned.swift

@@ -24,6 +24,9 @@ import VersionedFMWK
   // CHECK-NEXT: - (nullable instancetype)initFormerlyFailableValue:(NSInteger)value OBJC_DESIGNATED_INITIALIZER;
 } // CHECK-NEXT: @end
 
+// Make sure we use forward declarations like we would for non-versioned names.
+// CHECK: @class InnerClass;
+
 // CHECK-LABEL: @interface UsesNestedClass
 @objc class UsesNestedClass : NSObject {
   // CHECK-NEXT: - (InnerClass * _Nullable)foo SWIFT_WARN_UNUSED_RESULT;