Consider S a pseudo match alias

Fixes tests/pos/i18211.scala, where type avoidance would cause `S[? <:
Int]` which would otherwise be consider an unreducible wild and lead to
Any which isn't <: Int.

[Cherry-picked fd7f451]

Author: dwijnand

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4557,6 +4557,7 @@ object Types extends TypeUtils {
      */
     def isUnreducibleWild(using Context): Boolean =
       tycon.isLambdaSub && hasWildcardArg && !isMatchAlias
+        && !(args.sizeIs == 1 && defn.isCompiletime_S(tycon.typeSymbol)) // S is a pseudo Match Alias
 
     def tryCompiletimeConstantFold(using Context): Type =
       if myEvalRunId == ctx.runId then myEvalued