[cxx-interop] implicitly imported CxxShim module should not force cli…

[hyp]

…ent modules to enable C++ interoperability

Fixes #65831

• Explanation: When C++ interop is enabled, 'CxxShim' module was imported without '@_implementationOnly' implicitly by the compiler. This made it impossible to import such module that used C++ interop, but didn't expose it in its public interface into another Swift module that didn't enable C++ interop. This fix allows for that now, so a regular Swift module can now import a module that uses C++ interop in its implementation but not its public interface.
• Scope: Swift's and C++ interop, implicit imports in frontend.
• Risk: Low. Only affects C++ interoperability and doesn't change behavior for building modules that use C++ interop.
• Testing: Swift unit tests.
• PR: [cxx-interop] implicitly imported CxxShim module should not force cli… #65875
• reviewed by: @egorzhdan

[hyp]

@swift-ci please test

[hyp]

@swift-ci please test source compatibility