PEP-448 unpacking in dict literals.

foo = {'a': 2}
bar = {**foo}

Previously, foo would incorrectly be added to the value type of bar.

N.B. this allows Ast::DictAst::keys to contain null values (for unpacked
dicts, where there is no key). This is necessary to determine which
values should be unpacked, because the value node has no different
properties.

No test regressions. Two new tests previously failed, now pass.

Differential Revision: https://phabricator.kde.org/D3867