[lit] Fix type error for parallelism groups

Summary:
Whereas rL299560 and rL309071 call `parallelism_groups.items()`, under the
assumption that `parallelism_groups` is a `dict` type, the default
parameter for that attribute is a `list`. Change the default to a
`dict` for type correctness.

This regression in the unit tests would have been caught if the
unit tests were being run continously. It also would have been caught
if the lit project used a Python type checker such as `mypy`.

Test Plan:
As per the instructions in `utils/lit/README.txt`, run the lit unit
test suite:

```
utils/lit/lit.py \
    --path /path/to/your/llvm/build/bin \
    utils/lit/tests
```

Verify that the test `lit :: unit/TestRunner.py` fails before applying this
patch, but passes once this patch is applied.

Reviewers: mgorny, rnk, rafael

Reviewed By: mgorny

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35878

llvm-svn: 309122