Fix #24991: Weaken narrowing for ==

[jack-williams]

With this PR narrowing for == is now weaker for numbers, booleans, and string.

Previously == assumed that the types could not be coerced to eachother; now we no longer prune a number, string, or boolean when comparing against a number, string, or boolean.

declare let x: number | string | boolean
declare let n: number;

if (x == n) {
    x; // previously x has type number; now x has type number | string | boolean
}

This solution is not sound: we still prune when comparing to a literal type. Examples

declare let x: number | string | boolean
declare let n: number;

if (x == 1) {
    x; // x still has type 1
}

This choice was made to reduce potential breaking changes. I'm open to tweaking the semantics based on RWC.

Fixes #24991

[RyanCavanaugh]

@typescript-bot test this

[typescript-bot]

Heya @RyanCavanaugh, I've started to run the extended test suite on this PR at 0fe03d6. You can monitor the build here. It should now contribute to this PR's status checks.

[RyanCavanaugh]

@typescript-bot test this new merge commit

[typescript-bot]

Heya @RyanCavanaugh, I've started to run the extended test suite on this PR at 6542d65. You can monitor the build here. It should now contribute to this PR's status checks.

[RyanCavanaugh]

RWC is clean