In this commit, we ignore the new test for `linkTimeIf`.

This commit is a forward port of the compiler changes in
scala-js/scala-js@5e842d8

When B <: A and C <: A, `linkTimeIf[A](cond) { B }{ C }` would
generate an `.asInstanceOf[A]` that casts the resulting value to a
supertype. However, this cast is redundant, as the linker will
prune one branch at link time, and the remaining expression is
already known to be a compatible type.

This commit eliminates the surrounding `.asInstanceOf` around the
`linkTimeIf[T]` when both `thenp` and `elsep` are known to be
subtypes of `T`. The optimizer already routinely performs this
optimization. However, that comes too late for the module instance
field alias analysis performed by `IncOptimizer`. In that case,
while the desugarer removes the `LinkTimeIf`, the extra
`AsInstanceOf` prevents aliasing the field. Removing the cast ahead
of time in the compiler allows field aliases to be recognized in
the presence of `LinkTimeIf`s.

This commit is a forward port of the Scala.js commit
scala-js/scala-js@9bb267c

We now recognize the shapes of varargs, deconstruct them, and emit
instead calls to Scala.js-specific runtime methods. Those methods
choose at link-time the best implementation for the target platform.

The changes in `genActualArgs` revealed that we cannot trust
`Symbol.isPrivate`. It uses `lastDenot.flagsUNSAFE` under the hood,
which returns absurd results if `lastDenot` happened to be accessed
within an `atPhase(...)` call. Therefore, we change those calls to
`sym.is(Private)` instead. `Symbol.is` always uses an up-to-date
denotation.

Another consequence of the optimization is that the string
representation of varargs seqs is altered in Scala.js. This is
consistent with the Scala 2 behavior. We generalize the check file
lookup of Vulpix to allow platform-dependent check files. We use
that mechanism to introduce Scala.js-specific versions of the
affected tests.

This commit is essentially a forward port of the Scala.js commit
scala-js/scala-js@24750cf
except that, in dotc, previously we did not do the optimization at
all.