Add a comment for the AddSwiftSourceKit.cmake change

Author: akyrtzi

tools/SourceKit/cmake/modules/AddSwiftSourceKit.cmake

@@ -428,6 +428,8 @@ macro(add_sourcekit_xpc_service name framework_target)
 
   add_dependencies(${framework_target} ${name})
 
+  # This is necessary to avoid having an rpath with an absolute build directory.
+  # Without this, such an rpath is added during build time and preserved at install time.
   set_target_properties(${name} PROPERTIES
                         BUILD_WITH_INSTALL_RPATH On
                         INSTALL_RPATH "@loader_path/../lib"