Pass system PYTHONPATH into lit-spawned shells

[JamesNagurne]

Summary

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
is set.

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
spawned shells.

llvm/test/lit.cfg.py

 # test_exec_root: The root path where tests should be run.
 config.test_exec_root = os.path.join(config.llvm_obj_root, 'test')
 
 # Tweak the PATH to include the tools dir.
 llvm_config.with_environment('PATH', config.llvm_tools_dir, append_path=True)
 
 # Propagate some variables from the host environment.
 llvm_config.with_system_environment(
-    ['HOME', 'INCLUDE', 'LIB', 'TMP', 'TEMP', 'ASAN_SYMBOLIZER_PATH', 'MSAN_SYMBOLIZER_PATH'])
+    ['HOME', 'INCLUDE', 'LIB', 'TMP', 'TEMP', 'ASAN_SYMBOLIZER_PATH', 'MSAN_SYMBOLIZER_PATH', 'PYTHONPATH'])
 
 
 # Set up OCAMLPATH to include newly built OCaml libraries.
 top_ocaml_lib = os.path.join(config.llvm_lib_dir, 'ocaml')
 llvm_ocaml_lib = os.path.join(top_ocaml_lib, 'llvm')
 
 llvm_config.with_system_environment('OCAMLPATH')
 llvm_config.with_environment('OCAMLPATH', top_ocaml_lib, append_path=True)