Discriminant property checks

[ahejlsberg]

This PR formalizes a stricter notion of what is considered a discriminant property. A property of a union type is considered a discriminant property if

• the type of the property is not identical in each of the constituent types, and
• the type of the property is a union of unit types.

Because a type guard for a discriminant property of a discriminated union object invalidates preceding type guards for other properties in the same object, it is important to reduce false positives when determining if a property is a discriminant.

Fixes #10169.

[RyanCavanaugh]

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