Make priority change warning messages stable

Make priority change warning messages stable under different orders of eligible.
We now always report the previously chosen alternative first and the new one second.

Author: odersky

tests/warn/i15264.check

@@ -0,0 +1,20 @@
+-- Warning: tests/warn/i15264.scala:48:26 ------------------------------------------------------------------------------
+48 |    val a = summon[A[Int]] // warn
+   |                          ^
+   |                          Given search preference for repro.A[Int] between alternatives
+   |                            (repro.exports.given_C_V : [V](using x$1: priority.Prio0): repro.C[V])
+   |                          and
+   |                            (repro.exports.given_A_V : [V](using x$1: priority.Prio2): repro.A[V])
+   |                          has changed.
+   |                          Previous choice          : the first alternative
+   |                          New choice from Scala 3.7: the second alternative
+-- Warning: tests/warn/i15264.scala:55:29 ------------------------------------------------------------------------------
+55 |    val a = summon[A[Q[Int]]] // warn
+   |                             ^
+   |                Given search preference for repro.A[repro.Q[Int]] between alternatives
+   |                  (repro.qcontext.gbq : [V](using p1: priority.Prio1)(using b: repro.B[V]): repro.B[repro.Q[V]])
+   |                and
+   |                  (repro.qcontext.gaq : [V](using p2: priority.Prio2)(using a: repro.A[V]): repro.A[repro.Q[V]])
+   |                has changed.
+   |                Previous choice          : the first alternative
+   |                New choice from Scala 3.7: the second alternative