utils/CmpDriver: add brief documentation to indicate what this does

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209268 91177308-0d34-0410-b5e6-96231b3b80d8

utils/CmpDriver

 #!/usr/bin/env python
 
+"""
+A simple utility that compares tool invocations and exit codes issued by
+compiler drivers that support -### (e.g. gcc and clang).
+"""
+
 import subprocess
 
 def splitArgs(s):