Pass system PYTHONPATH into lit-spawned shells
lit generates script files and executes them via '/bin/bash <script>'.
This does not pass down environment variables unless they are explicitly
set by the test configuration.
In this example, there are a number of tests for the 'opt-viewer' utility
that require the pygments module. The check for these modules is done
during LLVM configuration (llvm/cmake/config-ix.cmake), where PYTHONPATH
Because PYTHONPATH is set in LLVM configuration, we find the pygments
module and set a cmake variable indicating that we can run the test.
Then, when we actually go to run the test, /bin/bash resets PYTHONPATH and
the test fails because it can no longer import pygments.
My proposed solution here is to pass PYTHONPATH down through lit into