Add a minimal C interpreter example.

 - Demonstrates how to build a standalone tool which loads source code using the
   Driver and Frontend libraries, and then uses CodeGen and the JIT to actually
   execute the code.

 - Still more complicated than it should be, but hey its only 153 lines. :)

--
ddunbar@ozzy:tmp$ cat hello.c
#include <stdio.h>
int main() { printf("hello world\n"); return 0; }
ddunbar@ozzy:tmp$ clang-interpreter hello.c
hello world
--

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97133 91177308-0d34-0410-b5e6-96231b3b80d8