Fix #4678: Create single Typed tree with multiple type annotations

[nicolasstucki]

This is mostly useful for printers as it represents what is written in the source.
I may also slightly improve performance of later phases.

With this change the tree becomes

result of tests/pos/i4678.scala after frontend:
package <empty> {
  class Foo() extends Object() { 
    val x: Int = 1:Int(1) @annot1 @annot2 @annot3 @annot4 @annot5
  }
  ...
}

instead of

result of tests/pos/i4678.scala after frontend:
package <empty> {
  class Foo() extends Object() { 
    val x: Int = 
      1:Int(1) @annot1:Int(1) @annot1 @annot2:Int(1) @annot1 @annot2 @annot3:
        Int(1) @annot1 @annot2 @annot3 @annot4
      :Int(1) @annot1 @annot2 @annot3 @annot4 @annot5
  }
  ...
}

[smarter]

Can we add a decompilation test to ensure the output is as expected ?

[nicolasstucki]

Added a decompilation test

[nicolasstucki]

I tried the definitions tests and it fails on both versions (before and after these changes). The trees in the annotated type do have the correct positions, nothing is lost with this change. I would fix that in a different PR as it is completely unrelated. Opened #5722 to track this issue.