[CMake] Update source file list only if different

Write source file list using 'file(GENERATE' so that they are updated
only if thier content is changed. Otherwise stdlib modules are rebuild
everytime cmake configuration happens.

Author: rintaro

stdlib/cmake/modules/SwiftSource.cmake

@@ -921,7 +921,7 @@ function(_compile_swift_files
   string(REPLACE ";" "'\n'" source_files_quoted "${source_files}")
   string(SHA1 file_name "'${source_files_quoted}'")
   set(file_path "${CMAKE_CURRENT_BINARY_DIR}/${file_name}.txt")
-  file(WRITE "${file_path}" "'${source_files_quoted}'")
+  file(GENERATE OUTPUT "${file_path}" CONTENT "'${source_files_quoted}'")
 
   # If this platform/architecture combo supports backward deployment to old
   # Objective-C runtimes, we need to copy a YAML file with legacy type layout