Allow nested private methods

pmd-java/etc/grammar/Java.jjt

@@ -1359,11 +1359,11 @@ void TypeDeclaration():
 
 void ClassOrInterfaceDeclaration(int modifiers):
 {
-Token t = null;
-jjtThis.setModifiers(modifiers);
+    Token t = null;
+    jjtThis.setModifiers(modifiers);
+    inInterface = false;
 }
 {
-
   ( /* See note about this optional final modifier in BlockStatement */
     ["final"|"abstract"] "class" | "interface" { jjtThis.setInterface(); inInterface = true; } )
   t=<IDENTIFIER> { jjtThis.setImage(t.image); }

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/ast/JDKVersionTest.java

@@ -218,6 +218,11 @@ public class JDKVersionTest {
         parseJava18("public class Foo { private void bar() { } }");
     }
 
+    @Test
+    public final void testNestedPrivateMethods() {
+        parseJava18("public interface Baz { public static class Foo { private void bar() { } } }");
+    }
+
     @Test
     public final void jdk9PrivateInterfaceMethods() {
         parseJava9(loadSource("jdk9_private_interface_methods.java"));