With this PR we improve the handling of structurally identical classes in union types and instanceof expressions:

• Structurally identical, but distinct, class types are now preserved in union types (instead of eliminating all but one).
• Union type subtype reduction only removes a class type if it is a subclass of and derives from another class type in the union.
• Type checking of the instanceof operator is now based on whether the type of the left operand derives from the type indicated by the right operand (as opposed to a structural subtype check).

This means that union types and instanceof properly distinguish between structurally identical classes. For example:

class A {}
class B extends A {}
class C extends A {}
class D extends A { c: string }
class E extends D {}
let x1 = !true ? new A() : new B();  // A
let x2 = !true ? new B() : new C();  // B | C (previously B)
let x3 = !true ? new C() : new D();  // C | D (previously C)
let a1 = [new A(), new B(), new C(), new D(), new E()];  // A[]
let a2 = [new B(), new C(), new D(), new E()];  // (B | C | D)[] (previously B[])
function f1(x: B | C | D) {
    if (x instanceof B) {
        x;  // B (previously B | D)
    }
    else if (x instanceof C) {
        x;  // C
    }
    else {
        x;  // D (previously never)
    }
}

The comments show the places where we previously got it wrong (or, I should say, less right).

Note that this PR does not change the fact that structurally identical types are subtypes and assignment compatible. The changes are entirely confined to the instanceof operator and reduction of union types.

The PR is technically a breaking change because we now preserve unions of structurally identical class types where we would previously reduce the union to one of the class types. Any errors resulting from this should be fixable with a type assertion or a type annotation. (There are no new errors compiling DefinitelyTyped, but a couple in the real world code suites.)

Fixes #10934.
Fixes #11965.
Fixes #14426.
Fixes #15585.
Fixes #15615.
Fixes #17612.
Fixes #18276.