Fix field count in emitted reflection data

PR swiftlang#78467 omitted certain fields from the FieldDescriptor list,
but did not update the count of fields that's emitted
just before that list.

Update the count so it matches the number of field descriptors
we actually emit.

Resolves rdar://143402921

Author: tbkka

Diff for: lib/IRGen/GenReflection.cpp

@@ -944,18 +944,35 @@ class FieldTypeMetadataBuilder : public ReflectionMetadataBuilder {
     B.addInt16(uint16_t(kind));
     B.addInt16(FieldRecordSize);
 
-    B.addInt32(getNumFields(NTD));
+    // Filter to select which fields we'll export FieldDescriptors for.
+    auto exportable_field =
+      [](Field field) {
+	// Don't export private C++ fields that were imported as private Swift fields.
+	// The type of a private field might not have all the type witness
+	// operations that Swift requires, for instance,
+	// `std::unique_ptr<IncompleteType>` would not have a destructor.
+	if (field.getKind() == Field::Kind::Var &&
+	    field.getVarDecl()->getClangDecl() &&
+	    field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
+	  return false;
+	// All other fields are exportable
+	return true;
+      };
+
+    // Count exportable fields
+    int exportableFieldCount = 0;
     forEachField(IGM, NTD, [&](Field field) {
-      // Skip private C++ fields that were imported as private Swift fields.
-      // The type of a private field might not have all the type witness
-      // operations that Swift requires, for instance,
-      // `std::unique_ptr<IncompleteType>` would not have a destructor.
-      if (field.getKind() == Field::Kind::Var &&
-          field.getVarDecl()->getClangDecl() &&
-          field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
-        return;
+      if (exportable_field(field)) {
+        ++exportableFieldCount;
+      }
+    });
 
-      addField(field);
+    // Emit exportable fields, prefixed with a count
+    B.addInt32(exportableFieldCount);
+    forEachField(IGM, NTD, [&](Field field) {
+      if (exportable_field(field)) {
+        addField(field);
+      }
     });
   }