Try enforcing contextual type of array literal elements

[RyanCavanaugh]

Part of #19236

Code

const s1: string[] = ["x", 3, null as any];
const s2: string[] = ["x", 3];

Expected behavior:

• s1 should be an error
• s2 should have an error span on 3

Actual behavior:

• s1 is not an error
• s2 has an error span on s2, which makes it impossible to figure out which element is wrong

Hypothesis: When checking a contextually-typed array literal, if an element type isn't assignable to the contextual type, issue an error on that element and return unknownType

[ahejlsberg]

@RyanCavanaugh On thing that would trip this up is overload resolution where we check the same expression multiple times with different contextual types.

[DanielRosenwasser]

Half of this is fixed. Thanks @weswigham.