fix: Closes #18367 -Wconf options are processed incorrectly

[Cherry-picked 16dd58f][modified]

Author: prolativ

compiler/src/dotty/tools/dotc/reporting/WConf.scala

@@ -119,7 +119,7 @@ object WConf:
   def fromSettings(settings: List[String]): Either[List[String], WConf] =
     if (settings.isEmpty) Right(WConf(Nil))
     else
-      val parsedConfs: List[Either[List[String], (List[MessageFilter], Action)]] = settings.map(conf =>
+      val parsedConfs: List[Either[List[String], (List[MessageFilter], Action)]] = settings.reverse.map(conf =>
         val filtersAndAction = conf.split(':')
         if filtersAndAction.length != 2 then Left(List("exactly one `:` expected (<filter>&...&<filter>:<action>)"))
         else

compiler/test/dotty/tools/dotc/config/ScalaSettingsTests.scala

@@ -89,7 +89,6 @@ class ScalaSettingsTests:
     val nowr = new Diagnostic.Warning("This is a problem.".toMessage, util.NoSourcePosition)
     assertEquals(Action.Silent, sut.action(nowr))
 
-  @Ignore("LTS backport rejected: https://github.com/scala/scala3/pull/18503")
   @Test def `i18367 rightmost WConf flags take precedence over flags to the left`: Unit =
     import reporting.{Action, Diagnostic}
     val sets = new ScalaSettings