Support Swift-Cxx reverse interop, bidirectional interop, and mixed Swift-Cxx targets

[maddyadams]

Description

In Xcode with Swift-Cxx interop enabled, developers can use:

• forward interop (importing C++ into Swift via a bridging header/modulemap)
• reverse interop (importing Swift into C++ via an autogenerated SwiftModule-Swift.h C++ header)
• mixed language targets (using both Swift and C++ source files in the same target)
• bidirectional interop (using both forward and reverse interop in the same mixed language target)

Currently, SwiftPM only supports forward interop in non-mixed language targets. It should improve feature parity by adding support for reverse interop, mixed language targets, and bidirectional interop.

Expected behavior

No response

Actual behavior

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Steps to reproduce

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Swift Package Manager version/commit hash

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

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[maddyadams]

rdar://137626621