[cxx-interop] Lower macOS deployment target version for the Cxx module

The Cxx module does not rely on any modern OS features. It can be deployed to older macOS versions.

rdar://117699474

Author: egorzhdan

stdlib/public/Cxx/CMakeLists.txt

@@ -24,6 +24,8 @@ add_swift_target_library(swiftCxx STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY IS
     # For functionality that depends on the C++ stdlib, use C++ stdlib overlay (`swiftstd` module).
     -Xcc -nostdinc++
 
+    DEPLOYMENT_VERSION_OSX ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_OSX}
+
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
     DEPENDS ${SWIFT_CXX_DEPS}
     INSTALL_IN_COMPONENT compiler

stdlib/public/Cxx/std/CMakeLists.txt

@@ -51,6 +51,8 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_O
     SWIFT_COMPILE_FLAGS_LINUX
     ${SWIFT_SDK_LINUX_CXX_OVERLAY_SWIFT_COMPILE_FLAGS}
 
+    DEPLOYMENT_VERSION_OSX ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_OSX}
+
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
     TARGET_SDKS ALL_APPLE_PLATFORMS LINUX WINDOWS
     INSTALL_IN_COMPONENT compiler