[utils] Add Check Compile Flow Consistency tool (check_cfc.py).

[utils] Add Check Compile Flow Consistency tool (check_cfc.py).

This is a tool for checking consistency of code generation with different
compiler options (such as -g or outputting to .s). This tool has found a number
of code generation issues. The script acts as a wrapper to clang or clang++
performing 2 (or more) compiles then comparing the object files. Instructions
for use are in check_cfc.py including how to use with LNT.

Differential Revision: http://reviews.llvm.org/D8723

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@233919 91177308-0d34-0410-b5e6-96231b3b80d8