Fix findFunctionType for OrTypes

Restore funtionality that was dropped in 6ac8b47

Fixes #15460

Author: odersky

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

@@ -1690,14 +1690,20 @@ object Types {
      *  is returned. If no function type is found, Any is returned.
      */
     def findFunctionType(using Context): Type = dealias match
-      case tp: AndOrType =>
+      case tp: AndType =>
         tp.tp1.findFunctionType & tp.tp2.findFunctionType
+      case tp: OrType =>
+        val tf1 = tp.tp1.findFunctionType
+        val tf2 = tp.tp2.findFunctionType
+        if !tf1.exists then tf2
+        else if !tf2.exists then tf1
+        else NoType
       case t if defn.isNonRefinedFunction(t) =>
         t
       case t @ SAMType(_) =>
         t
       case _ =>
-        defn.AnyType
+        NoType
 
     /** This type seen as a TypeBounds */
     final def bounds(using Context): TypeBounds = this match {

tests/pos/i15460.scala

@@ -0,0 +1,5 @@
+type C = (() => Int) | (() => String)
+
+def foo(c: C): Unit = ()
+
+val _ = foo(() => 1)