Improve report of erroneous sequence argument

[som-snytt]

Minimized code

➜  dotty git:(master) ./bin/scala
Starting scala3 REPL...
scala> List(1, List(2): _*)
val res0: List[Any] = List(1, List(2))

Output

Sometimes the missing error on the sequence argument ascription makes it hard to spot the problem.

scala> import collection.mutable.ListBuffer

scala> val xs = ListBuffer.empty[Int]
val xs: scala.collection.mutable.ListBuffer[Int] = ListBuffer()

scala> xs.+=(1, 2, 3, List(4,5,6) ++ List(7,8,9): _*)
1 |xs.+=(1, 2, 3, List(4,5,6) ++ List(7,8,9): _*)
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |               Found:    List[Int]
  |               Required: Int

scala> xs.+=(1, 2, List(4,5,6) ++ List(7,8,9): _*)
1 |xs.+=(1, 2, List(4,5,6) ++ List(7,8,9): _*)
  |^^^^^
  |method += in trait Growable is deprecated since 2.13.0: Use `++=` aka `addAll` instead of varargs `+=`; infix operations with an operand of multiple args will be deprecated
val res0: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 4, 5, 6, 7, 8, 9)

Expectation

➜  dotty git:(master) scala
Welcome to Scala 2.13.3 (OpenJDK 64-Bit Server VM, Java 14.0.2).
Type in expressions for evaluation. Or try :help.

scala> List(1, List(2): _*)
                      ^
       error: no `: _*' annotation allowed here
       (such annotations are only allowed in arguments to *-parameters)

This is a follow-up to #9749 inspired by #10062

[som-snytt]

Duplicated by #11419 fixed by #11423