gn build: Copy file permissions from input file in configure_file() emulation.

Most significantly, this makes bin/llvm-lit executable so that it
can be run in the usual way.

Differential Revision: https://reviews.llvm.org/D56423

llvm-svn: 350688

llvm/utils/gn/build/write_cmake_config.py

@@ -101,6 +101,7 @@ def main():
 
     if not os.path.exists(args.output) or open(args.output).read() != output:
         open(args.output, 'w').write(output)
+        os.chmod(args.output, os.stat(args.input).st_mode & 0777)
 
 
 if __name__ == '__main__':