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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350688 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/build/write_cmake_config.py b/utils/gn/build/write_cmake_config.py
index 0f47654..f0d0a00 100755
--- a/utils/gn/build/write_cmake_config.py
+++ b/utils/gn/build/write_cmake_config.py
@@ -101,6 +101,7 @@
 
     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__':