classfile reader: allow CONSTANT_Dynamic in constant pool

forward-port of scala/scala#10675
references scala/bug#12396
fixes scala#19527

compiler/src/dotty/tools/dotc/core/classfile/ClassfileConstants.scala

@@ -73,6 +73,7 @@ object ClassfileConstants {
 
   inline val CONSTANT_METHODHANDLE  = 15
   inline val CONSTANT_METHODTYPE    = 16
+  inline val CONSTANT_DYNAMIC       = 17
   inline val CONSTANT_INVOKEDYNAMIC = 18
 
   // tags describing the type of a literal in attribute values

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -112,7 +112,7 @@ object ClassfileParser {
             in.skip(3)
           case CONSTANT_FIELDREF | CONSTANT_METHODREF | CONSTANT_INTFMETHODREF
              | CONSTANT_NAMEANDTYPE | CONSTANT_INTEGER | CONSTANT_FLOAT
-             | CONSTANT_INVOKEDYNAMIC =>
+             | CONSTANT_INVOKEDYNAMIC | CONSTANT_DYNAMIC =>
             in.skip(4)
           case CONSTANT_LONG | CONSTANT_DOUBLE =>
             in.skip(8)

tests/pos/t12396/A_1.java

@@ -0,0 +1,17 @@
+// test: -jvm 21+
+
+public class A_1 {
+  public int f(Object s) {
+    switch(s) {
+      case Res.R -> {
+        return 1;
+      }
+      default -> {
+        return 3;
+      }
+    }
+  }
+  static enum Res {
+    R
+  }
+}

tests/pos/t12396/B_2.scala

@@ -0,0 +1,5 @@
+// test: -jvm 21+
+
+class B {
+  def bar = (new A_1).f(null)
+}