Keep annotation order

This change makes sure non-repeated annotations are kept in the order
they were found in the source code.

The motivation is not necessarily to have them in the original order,
but to have them in an order that is deterministic across rebuilds
(potentially even across different machines), for reasons discussed
further in scala#7661 and the corresponding scala/scala-dev#405

I tried adding an 'integration test' in `tests/pos` to be picked up by
`IdempotencyCheck.scala`, but unfortunately couldn't reproduce the
nondeterminism that way, so didn't end up including it in this commit.

I didn't see an obvious place for a 'unit test' of this code, I'd be
happy to add one when someone can recommend a good place to put it.

This is basically the dotty equivalent of
scala/scala@954c5d3

Fixes scala#14743

compiler/src/dotty/tools/dotc/transform/RepeatableAnnotations.scala

@@ -11,6 +11,8 @@ import Constants._
 import Types._
 import Decorators._
 
+import scala.collection.mutable
+
 class RepeatableAnnotations extends MiniPhase:
 
   override def phaseName: String = RepeatableAnnotations.name
@@ -28,8 +30,8 @@ class RepeatableAnnotations extends MiniPhase:
     tree
 
   private def aggregateAnnotations(annotations: Seq[Annotation])(using Context): List[Annotation] =
-    val annsByType = annotations.groupBy(_.symbol)
-    annsByType.flatMap {
+    val annsByType = stableGroupBy[Annotation, Symbols.Symbol](annotations, _.symbol)
+    annsByType.flatMap[Annotation] {
       case (_, a :: Nil) => a :: Nil
       case (sym, anns) if sym.derivesFrom(defn.ClassfileAnnotationClass) =>
         sym.getAnnotation(defn.JavaRepeatableAnnot).flatMap(_.argumentConstant(0)) match
@@ -50,6 +52,14 @@ class RepeatableAnnotations extends MiniPhase:
       case (_, anns) => anns
     }.toList
 
+  private def stableGroupBy[A, K](ins: Seq[A], f: A => K): scala.collection.MapView[K, List[A]] =
+    val out = new mutable.LinkedHashMap[K, mutable.ListBuffer[A]]()
+    for (in <- ins) {
+      val buffer = out.getOrElseUpdate(f(in), new mutable.ListBuffer)
+      buffer += in
+    }
+    out.view.mapValues(_.toList)
+
 object RepeatableAnnotations:
   val name: String = "repeatableAnnotations"
   val description: String = "aggregate repeatable annotations"