[cmake] Now that we link with cxx and set the right load paths, use appropriate backdeployment target for swiftc.

Before a8ae952, we could not do this since we
would fail to link since we didn't pass to clang the path to the toolchain dir
when the compatibility libraries live.

Author: gottesmm

cmake/modules/AddSwift.cmake

@@ -92,7 +92,14 @@ function(_add_host_variant_c_compile_link_flags name)
       MACCATALYST_BUILD_FLAVOR ""
       DEPLOYMENT_VERSION "${DEPLOYMENT_VERSION}")
     target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
-    target_link_options(${name} PRIVATE -target;${target})
+    target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
+  endif()
+
+  if (CMAKE_Swift_COMPILER)
+    get_target_triple(target target_variant "${SWIFT_HOST_VARIANT_SDK}" "${SWIFT_HOST_VARIANT_ARCH}"
+      MACCATALYST_BUILD_FLAVOR ""
+      DEPLOYMENT_VERSION "${DEPLOYMENT_VERSION}")
+    target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:Swift>:-target;${target}>)
   endif()
 
   set(_sysroot