feat(compiler): allow selector-less directives as base classes

[alxhub]

In Angular today, the following pattern works:

export class BaseDir {
  constructor(@Inject(ViewContainerRef) protected vcr: ViewContainerRef) {}
}

@Directive({
  selector: '[child]',
})
export class ChildDir extends BaseDir {
  // constructor inherited from BaseDir
}

A decorated child class can inherit a constructor from an undecorated base
class, so long as the base class has metadata of its own (for JIT mode).
This pattern works regardless of metadata in AOT.

In Angular Ivy, this pattern does not work: without the @directive
annotation identifying the base class as a directive, information about its
constructor parameters will not be captured by the Ivy compiler. This is a
result of Ivy's locality principle, which is the basis behind a number of
compilation optimizations.

As a solution, @directive() without a selector will be interpreted as a
"directive base class" annotation. Such a directive cannot be declared in an
NgModule, but can be inherited from.

[alxhub]

Presubmit

[kara]

LGTM, one nit

[kara]

Nit: Can we flip the ternary so it's not "if-not/else" ?

           if (metadataResolver.isAbstractDirective(symbol)) {
              abstractDirectives.push(symbol);
            } else {
              directives.push(symbol);
            }

[alxhub]

The general rule I try to use in the compiler is that for simple if-else blocks, the common case is the first branch. So in this case, most directives are plain directives and flow through the if section, and the else section handles the occasional abstract directive. That does make the conditional a little trickier to parse, though.

I added some extra comments to clarify the two bodies here :)

[IgorMinar]

lgtm for api changes

[alxhub]

Caretaker: reviews in place

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