lit: Add 'cd' support to the internal shell and port some tests

lit: Add 'cd' support to the internal shell and port some tests

The internal shell was already threading around a 'cwd' parameter. We
just have to make it mutable so that we can update it as the test script
executes.

If the shell ever grows support for environment variable substitution,
we could also implement support for export.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231017 91177308-0d34-0410-b5e6-96231b3b80d8