Merge pull request #21890 from DougGregor/dylib-compatibiilty-version

Always set the compatibility_version of linked dylibs on Darwin.

Author: DougGregor

cmake/modules/AddSwift.cmake

@@ -755,9 +755,10 @@ function(_add_swift_library_single target name)
     endif()
   endif()
 
-  if (SWIFT_COMPILER_VERSION)
-    if(${SWIFTLIB_SINGLE_SDK} IN_LIST SWIFT_APPLE_PLATFORMS)
-      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-current_version" "-Xlinker" "${SWIFT_COMPILER_VERSION}" "-Xlinker" "-compatibility_version" "-Xlinker" "1")
+  if(${SWIFTLIB_SINGLE_SDK} IN_LIST SWIFT_APPLE_PLATFORMS)
+    list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-compatibility_version" "-Xlinker" "1")
+    if (SWIFT_COMPILER_VERSION)
+      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-current_version" "-Xlinker" "${SWIFT_COMPILER_VERSION}" )
     endif()
   endif()