lit: Add internal script execution.

 - Off by default, you can test it with the --no-sh argument.

 - For me it works for all but 3 tests, but there a number of FIXMEs and QOI
   issues:  
     o Redirection isn't completely accurate -- in practice it can't portably
       be, but I would like to error out if someone writes something which isn't
       going to work. This is the source of the 3 test failures.

     o Some pipe configurations have the potential to deadlock.

     o It is significantly slower when multithreaded. I believe this is due to
       locking happening under the hood, there is probably some kind of solution
       but I haven't investigated yet.

     o Log output is ugly.

llvm-svn: 77784