this-property assignments should use autoType to get control flow narrowing

[sandersn]

Based on discussion in #37827, we could further improve typing of this-property assignments if we used autoType whenever we can't tell what type they should have.

This has three open questions:

1. Performance.
2. Is there benefit in real code? This only helps if (1) we aren't getting the right type some of time where (2) the auto type would get the right type. This is likely to be when a property is used repeatedly in a long method.
3. What changes are actually needed. Just returning autoType doesn't work, because control flow narrowing doesn't seem to track this-property assignments as references.

[sandersn]

implementation hiccup number 1 is that a union with auto turns into any, not auto.

[ahejlsberg]

I think you could do this by utilizing logic similar to that in checkPropertyInitialization where we check that properties are definitely assigned in the constructor. In particular, the control flow walk we do in isPropertyInitializedInConstructor where we start from a virtual flow node representing all constructor return points. If you were to pass in autoType for the declared type and undefinedType for the initial type, the result should be a CFA'd aggregate of the possible assigned values upon return from the constructor. You'd furthermore have to special case this.xxx property access expressions inside the constructor to similarly use autoType and undefinedType for declared and initial types (which would allow this.x = this.x + ... without circularity errors). But for references outside the constructor you would rely on the CFA computed type.

[ahejlsberg]

@sandersn I've put up an experimental prototype of this in #37920.