Fix EnclosingMethod for lifted anonfun

The anonfun "() => new TB {.." code is lifted to a static method, in the
original class (A), but the GenBCode logic was still returning the TA
anon class.

compiler/src/dotty/tools/backend/jvm/BCodeAsmCommon.scala

@@ -76,7 +76,9 @@ final class BCodeAsmCommon[I <: DottyBackendInterface](val interface: I) {
       if (sym.isClass) sym
       else enclosingClass(sym.originalOwner.originalLexicallyEnclosingClass)
     }
-    enclosingClass(classSym.originalOwner.originalLexicallyEnclosingClass)
+    enclosingMethodForEnclosingMethodAttribute(classSym) match
+      case Some(methodSym) => methodSym.skipLocalOwners.owner
+      case _ => enclosingClass(classSym.originalOwner.originalLexicallyEnclosingClass)
   }
 
   /*final*/ case class EnclosingMethodEntry(owner: String, name: String, methodDescriptor: String)

tests/run/i18701.check

@@ -0,0 +1 @@
+public static final TB A.A$$anon$1$$_$_$$anonfun$1()

tests/run/i18701.fixed.check

@@ -0,0 +1 @@
+public TB A$$anon$2.apply()

tests/run/i18701.fixed.scala

@@ -0,0 +1,15 @@
+abstract class TA { def tb(): TB }
+abstract class TB { def chk(): Unit }
+class A:
+  def a(): TA =
+    new TA {
+      def tb(): TB =
+        val fn: () => TB = new Function0[TB]:
+          def apply(): TB = new TB {
+            def chk() = println(getClass.getEnclosingMethod())
+          }
+        fn()
+    }
+
+object Test:
+  def main(args: Array[String]): Unit = new A().a().tb().chk()

tests/run/i18701.scala

@@ -0,0 +1,14 @@
+abstract class TA { def tb(): TB }
+abstract class TB { def chk(): Unit }
+class A:
+  def a(): TA =
+    new TA {
+      def tb(): TB =
+        val fn: () => TB = () => new TB {
+          def chk() = println(getClass.getEnclosingMethod())
+        }
+        fn()
+    }
+
+object Test:
+  def main(args: Array[String]): Unit = new A().a().tb().chk()