[cxx-interop] Flip the default value of `SWIFT_ENABLE_EXPERIMENTAL_CXX_INTEROP`
This changes the default value of the CMake flag SWIFT_ENABLE_EXPERIMENTAL_CXX_INTEROP from FALSE to TRUE.
C++ interop is an important part of the compiler. If necessary, it's still possible to disable it explicitly.
@swift-ci please test
@swift-ci please build toolchain Windows
@swift-ci please test macOS
@swift-ci please smoke test macOS
@swift-ci please smoke test macOS
@swift-ci please smoke test macOS
The macOS test failure is unrelated, I'm merging this.