[cxx-interop] Avoid spurious type aliases in reverse interop

To support nested structs, we emit type aliases in the outer class.
Unfortunately, we emitted these type aliases unconditionally, even if
the actualy nested struct was not emitted to the reverse interop header
(due to visibility or the construct being unsupported). This PR fixed
this issue by checking first if the nested entity should be included in
the reverse interop header.

rdar://141688074

Author: Gabor Horvath

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -257,6 +257,9 @@ class DeclAndTypePrinter::Implementation
       for (const Decl *member : members) {
         if (member->getModuleContext()->isStdlibModule())
           break;
+        auto VD = dyn_cast<ValueDecl>(member);
+        if (!VD || !shouldInclude(VD))
+          continue;
         // TODO: support nested classes.
         if (isa<ClassDecl>(member))
           continue;

test/Interop/SwiftToCxx/structs/nested-structs-in-cxx.swift

@@ -60,3 +60,9 @@ public func makeRecordConfig() -> RecordConfig {
 public func makeAudioFileType() -> AudioFileType {
     return AudioFileType.CAF(AudioFileType.SubType(id: 42))
 }
+
+public class TestObject {
+    enum CustomError: Swift.Error {
+        case invalid
+    }
+}