[C++ interop] c++ language standard is not passed to Swift's clang importer when C++ interop is enabled

[hyp]

Description

C++ interoperability can be enabled using the new .interoperabilityMode(.Cxx) setting.
Such Swift target should be able to import any C++ headers, e.g. If the user specifies that the package manifest uses C++20 mode then it should be possible to import C++ code that requires C++20. SwiftPM should pass -Xcc -std=c++20 in that case.

Expected behavior

SwiftPM should pass -Xcc -std=c++20 in that case, when a Swift target enables C++ interop, and the package manifest specifies that C++20 is used.

Actual behavior

SwiftPM doesn't pass C++20 language mode to clang importer in Swift.

Steps to reproduce

No response

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version ; uname -a)

No response