Adding `import CxxStdlib` makes diagnostic about not enabling C++ interoperability harder to understand

[maddyadams]

Description

If module A enables Swift-Cxx interoperability, module B does not enable Swift-Cxx interoperability, and B imports A, then the compiler emits the error message:
Module 'A' was built with C++ interoperability enabled, but current compilation does not enable C++ interoperability.

But, if A imports CxxStdlib, the error message becomes harder to understand and resolve:
<unknown>:0: error: cannot load underlying module for 'CxxStdlib'

Reproduction

// A.swift, Swift-Cxx interop enabled
import Foundation
import CxxStdlib

// B.swift, Swift-Cxx interop disabled
import A

Expected behavior

I expected that the compiler would emit the error Module 'A' was built with C++ interoperability enabled, but current compilation does not enable C++ interoperability

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

Additional information

rdar://139033386