fix: Ignore assignment to non-identifier/literals

[yungsters]

Currently, react-docgen fatals on components assigned to non-identifier/literals. For example:

Foo.Bar = () => {};
// TypeError: Argument must be an Identifier or a Literal

This fixes it by checking the left expression of the assignment before attempting to extract its name.

Test Plan:

yarn test
yarn flow
yarn lint

[danez]

Thanks