fix: Allow as as an infix type in non context bound types

Author: KacperFKorban

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -4467,4 +4467,4 @@ object Parsers {
       (EmptyValDef, List(EmptyTree))
     }
   }
-}
+}

tests/pos/i21769.scala

@@ -0,0 +1,10 @@
+
+infix trait as[From, To]
+
+val conv: (String as Int) = ???
+given instance: (String as Int) = ???
+def test(ev: (String as Int)) = ???
+
+class F
+
+class K extends (F as K)