Rework recent, buggy change to immutable.TreeMap

[retronym]

We need to favour keys from the left that equivalent (but ne),
while favouring values from the right.

Revert then rework aa8bf99

Fixes scala/bug#12042 (at the second attempt)

[retronym]

Thread safety bug in scala-reflect?

https://scala-ci.typesafe.com/job/scala-2.13.x-validate-main/12271/testReport/junit/test.files/run_/reflection_valueclasses_derived_scala/

!    1 - run/reflection-valueclasses-derived.scala  [output differs]
% diff /home/jenkins/workspace/scala-2.13.x-validate-main/test/files/run/reflection-valueclasses-derived.check /home/jenkins/workspace/scala-2.13.x-validate-main/test/files/run/reflection-valueclasses-derived-run.log
@@ -1,3 +1,27 @@
+Uncaught exception on thread Thread[Reflector-61,5,main]: java.lang.IndexOutOfBoundsException: 2 is out of bounds (min 0, max 0)
+java.lang.IndexOutOfBoundsException: 2 is out of bounds (min 0, max 0)
+	at scala.collection.mutable.ArrayBuffer.apply(ArrayBuffer.scala:97)
+	at scala.reflect.internal.tpe.TypeConstraints.containsSymbol(TypeConstraints.scala:205)
+	at scala.reflect.internal.tpe.TypeConstraints.solveOne$1(TypeConstraints.scala:256)
+	at scala.reflect.internal.tpe.TypeConstraints.$anonfun$solve$8(TypeConstraints.scala:290)
+	at scala.reflect.internal.tpe.TypeConstraints.solve(TypeConstraints.scala:290)
+	at scala.reflect.internal.tpe.TypeConstraints.solve$(TypeConstraints.scala:218)
+	at scala.reflect.internal.SymbolTable.solve(SymbolTable.scala:28)
+	at scala.reflect.internal.Types$ExistentialType.withTypeVars(Types.scala:3209)
+	at scala.reflect.internal.tpe.TypeComparers.thirdTry$1(TypeComparers.scala:553)
+	at scala.reflect.internal.tpe.TypeComparers.secondTry$1(TypeComparers.scala:520)
+	at scala.reflect.internal.tpe.TypeComparers.firstTry$1(TypeComparers.scala:496)
+	at scala.reflect.internal.tpe.TypeComparers.isSubType2(TypeComparers.scala:616)
+	at scala.reflect.internal.tpe.TypeComparers.isSubType1(TypeComparers.scala:348)
+	at scala.reflect.internal.tpe.TypeComparers.isSubType(TypeComparers.scala:306)
+	at scala.reflect.internal.tpe.TypeComparers.isSubType$(TypeComparers.scala:268)
+	at scala.reflect.internal.SymbolTable.isSubType(SymbolTable.scala:28)
+	at scala.reflect.internal.Types$Type.$less$colon$less(Types.scala:814)
+	at scala.reflect.internal.Types$Type.$less$colon$less(Types.scala:270)
+	at Test$.$anonfun$tasks$1(reflection-sync-subtypes.scala:7)
+	at Test$$anon$1.$anonfun$run$1(reflection-sync-subtypes.scala:15)
+	at Test$$anon$1.$anonfun$run$1$adapted(reflection-sync-subtypes.scala:15)
+	at Test$$anon$1.run(reflection-sync-subtypes.scala:15)

[lrytz]

I removed the change to reusable instances; that issue seems unrelated to this PR, and it was causing NPEs (which would probably be fixed with lazy vals).

[SethTisue]

TODO:

• Add a generic test case to SetMapRulesTest that would have caught the bug before we found it in scala-collections-laws.

improving the tests can happen post-release; I've made a separate ticket on it at scala/scala-dev#713

[retronym]

I removed the change to reusable instances; that issue seems unrelated to this PR, and it was causing NPEs (which would probably be fixed with lazy vals).

#9080