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