Template literal types for contextually typed template literal expressions

[ahejlsberg]

In #41891 we introduced template literal types for all template literal expressions. That turned out to be too much of a breaking change, and it was reverted in #42588. In this PR we instead introduce template literal types for contextually typed template literal expressions. Specifically, a template literal expression is given a template literal type if it is contextually typed by a string literal or template literal type, or if it is contextually typed by a generic type with a constraint that is assignable to type string, alleviating the need to use as const in scenarios where it is obvious a template literal type is desired. This less aggressive approach fixes the originally reported issues and shouldn't cause any backwards compatibility breaks.

Some examples:

function bar(s: string): `hello ${string}` {
    return `hello ${s}`;  // Now ok, previously was error
}

declare let s: string;
declare function g1<T>(x: T): T;
declare function g2<T extends string>(x: T): T;

let x1 = g1(`xyz-${s}`);  // string
let x2 = g2(`xyz-${s}`);  // `xyz-${string}`, previously was string

Fixes #43143.

[orta]

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[typescript-bot]

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