[FixCode] Apply coercion fixit for assignment types' mismatch too

[benlangmuir]

What's in this pull request?

• Explanation: We have a fixit for adding casts for type mismatches, but it's not being applied in assignments, which negatively impacts migration to swift-3.
• Scope: Affects users assigning Any values into AnyObject variables. This is a fairly common migration situation after the id-as-Any changes.
• Risk: Very low; just adds a switch case that was missed to an existing fixit.
• Reviewed by: Ben Langmuir (patch by Xi with test fixup by Argyrios)
• Testing: Regression test added.

Resolved bug number: (SR-)

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[benlangmuir]

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