Cache anonymous object type instantiations

[ahejlsberg]

Fixes #4443.

This PR adds a cache to TypeMapper which caches instantiations of anonymous object types previously created using that type mapper. The PR supercedes #4456.

[ahejlsberg]

Tests copied from #4456.

[DanielRosenwasser]

Can you also add this test?

tests/cases/compiler/typeArgumentInferenceWithRecursivelyReferencedTypeAliasToTypeLiteral03.ts

type TreeNode = {
    leftRight: [TreeNode, TreeNode];
};

function foo<U>(x: TreeNode) { }
var n: TreeNode;
foo(n);

[ahejlsberg]

@DanielRosenwasser Don't think we need that test. It doesn't really test anything new, rather it just reveals an issue with the solution in #4456 that doesn't apply here.

[DanielRosenwasser]

👍

[JsonFreeman]

👍
I remember when we tried this before, but dismissed it because it didn't work for infinitely expanding anonymous types. But it sure works for recursive types.

[DanielRosenwasser]

@JsonFreeman what are the cases in which it didn't work?

[JsonFreeman]

@DanielRosenwasser, we fixed those already. See PR #3316 and #3452 (there are links to related issues there).

[DanielRosenwasser]

@ahejlsberg can you cherry pick this into release-1.6 if it hasn't been done already?