Normally, annotations applied to a class have no business being
replicated on a synthetic companion object. One exception was the
`@alpha` method, which is supposed to apply as well. Instead of
trying to identify those annotations without the symbols during
`Desugar`, we change `erasedName` to go look on the companion class
of synthetic module classes.

The `PrepJSInterop` phase is responsible for:

* Performing all kinds of Scala.js-specific compile-time checks,
  and emitting the appropriate compile errors.
* Perform some transformations that are necessary for JavaScript
  interop, notably generating exports forwarders.

This commit ports all the functionality of `PrepJSInterop` from
Scala 2, except the following:

* Handling of `scala.Enumeration`s: it is unclear whether we still
  want to support that in the core, or if it should be handled by
  an optional compiler plugin in the future.
* Exports: they will be done later.
* Warnings about duplicate fields in `js.Dynamic.literal`: mostly
  because they are non-essential.

The test cases are ported from the Scala.js compiler tests.

This is a forward port of the upstream commit
scala-js/scala-js@3c3b100

We now require the option
`-scalajs-genStaticForwardersForNonTopLevelObjects` to emit static
forwarders for non-top-level objects.

In addition, we take `-XnoForwarders` into account, which we were
erroneously not doing before.

This is a forward port of the upstream commit
scala-js/scala-js@6c82635

Since dotc does not support warning suppression yet, we use a
regular warning to report when we avoid emitting static forwarders.

And enable a number of tests that have been updated upstream to be
compatible with dotty.

…base

This is a forward port of the back-end changes in the upstream
commit
scala-js/scala-js@a1a27f2

This is very important, as it prevents `SelectStatic` from creating
dangling references to global scope objects. Such dangling
references are invalid and trigger an error in the back-end.