[CMake] Handle cases where Swift parser integration is disabled in Linux

When Swift parser integration is disabled (i.e. NOT SWIFT_SWIFT_PARSER),
macro plugins targets are not created. So CMake should bail out.

(cherry picked from commit ad2dacd)
(cherry picked from commit 5125ac3)

Author: rintaro

lib/Macros/CMakeLists.txt

@@ -31,6 +31,12 @@ function(add_swift_macro_library name)
   # Add the library.
   add_pure_swift_host_library(${name} SHARED ${ASML_SOURCES})
 
+  # If we don't have the Swift swift parser, bail out, because the above
+  # add_pure_swift_host_library did nothing.
+  if (NOT SWIFT_SWIFT_PARSER)
+    return()
+  endif()
+
   # Add rpath to 'lib/{platform}'
   file(RELATIVE_PATH relpath_to_lib
     "${SWIFT_HOST_PLUGINS_DEST_DIR}"
@@ -44,12 +50,6 @@ function(add_swift_macro_library name)
       APPEND PROPERTY INSTALL_RPATH "$ORIGIN/..")
   endif()
 
-  # If we don't have the Swift swift parser, bail out, because the above
-  # add_pure_swift_host_library did nothing.
-  if (NOT SWIFT_SWIFT_PARSER)
-    return()
-  endif()
-
   # Install into the plugin directory.
   set_target_properties(${name}
     PROPERTIES