Test for swiftlang#78981

**Without swiftlang#78981**: This test triggers "Reflection section too small to contain next record"
errors, indicating that the field information is broken.

**With swiftlang#78981**: This test runs to completion without triggering
the error message above.

Author: tbkka

validation-test/Reflection/reflect_Cxx.swift

@@ -0,0 +1,59 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// %target-clangxx -c %S/Inputs/Cxx/pimpl.cpp -I %S/Inputs/Cxx -o %t/pimpl.o
+// RUN: %target-build-swift -lswiftSwiftReflectionTest %t/reflect_Cxx.swift -I %t -cxx-interoperability-mode=default -o %t/reflect_Cxx
+// RUN: %target-codesign %t/reflect_Cxx
+
+// RUN: %target-run %target-swift-reflection-test %t/reflect_Cxx | tee /dev/stderr | %FileCheck %s
+
+// REQUIRES: reflection_test_support
+// REQUIRES: executable_test
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: asan
+
+//--- module.modulemap
+
+module CxxStruct {
+  header "CxxStruct.h"
+  requires cplusplus
+  export *
+}
+
+//--- CxxStruct.h
+
+struct CxxStructA {
+private:
+  int i;
+};
+
+inline CxxStructA createCxxStructA() {
+  return CxxStructA();
+}
+
+//--- reflect_Cxx.swift
+
+import SwiftReflectionTest
+import CxxStruct
+
+// Trigger type metadata to be emitted by conforming C++ types to a Swift protocol.
+protocol MyProto {}
+extension CxxStructA : MyProto {}
+
+let a = createCxxStructA()
+reflect(any: a)
+
+// CHECK: Reflecting an existential.
+// CHECK-NEXT: Instance pointer in child address space: 0x{{[0-9a-fA-F]+}}
+// CHECK-NEXT: Type reference:
+// CHECK-NEXT: (struct __C.CxxStructA)
+
+// CHECK: Type info:
+// CHECK-NEXT: (builtin size=4 alignment=4 stride=4 num_extra_inhabitants=0 bitwise_takable=1)
+// CHECK-NEXT: Mangled name: $sSo10CxxStructAV
+// CHECK-NEXT: Demangled name: __C.CxxStructA
+
+// CHECK: Start of instance data: 0x{{[0-9a-fA-F]+}}
+
+doneReflecting()
+
+// CHECK: Done.