Fix weak object relation check

[ahejlsberg]

This PR optimizes two code paths:

• Weak object checks now exclude generic types and therefore never fetch constraints. Previously, for a type variable constrained by a union type we'd end up computing the property set of the union type which can be an expensive operation. Also, we'd do this before checking whether the target is actually a weak type (a relatively cheap check). Now we do it the other way around.
• Type relationship checks with a tuple on the source side and an array on the target side are now specialized to just check the type of the index signature. This improves performance and error elaborations.

Fixes #26756.

[ahejlsberg]

@typescript-bot test this

[typescript-bot]

Heya @ahejlsberg, I've started to run the extended test suite on this PR at a35c496. You can monitor the build here. It should now contribute to this PR's status checks.

[sandersn]

I looked for explicit tuple-tuple relations in isRelatedTo and couldn't find it. Can you point me to it?

[ahejlsberg]

Right here https://github.com/Microsoft/TypeScript/pull/26790/files#diff-c3ed224e4daa84352f7f1abcd23e8ccaR11832

[mihailik]

Type relationship checks with a tuple on the source side and an array on the target side are now specialized to just check the type of the index signature.

What if the array side is more specific than just array?

interface TupleOfMillion extends Array<number> {
    0: 1000000;
    length: 1;
}

var tmillion: TupleOfMillion, tten: [10];

tmillion = tten; // Type '10' is not assignable to type '1000000'.
tten = tmillion; // Type '1000000' is not assignable to type '10'.

[ahejlsberg]

@mihailik The optimization only applies to checks between tuples and Array<T> or ReadonlyArray<T> instantiations. All other types are related structurally as usual.