Bug fix. Correctly identify expressions using &=, ^=, >>=, and <<= as assignment statements when parsing BUILD files. (#1357)

* Add files via upload

Correctly include &=, ^=, >>=, <<= as assignment statements when parsing BUILD files.

The Starlark grammar reference (https://github.com/bazelbuild/starlark/blob/master/spec.md#grammar-reference) identifies all of these as assignment statements, but the parser doesn't include them in the list, probably an old oversight.

This fixes issue #1330.

* Update warn_control_flow_test

This change adds no-effect tests for &=, ^=, >>=, and <<= to ensure they are recognized as having an effect.