8199852: Print more information about class loaders in LinkageErrors

Reviewed-by: dholmes, lfoltan, gtriantafill

src/hotspot/share/classfile/javaClasses.hpp

@@ -1321,6 +1321,12 @@ class java_lang_ClassLoader : AllStatic {
   // Debugging
   friend class JavaClasses;
   friend class ClassFileParser; // access to number_of_fake_fields
+
+  // Describe ClassLoader for exceptions, tracing ...
+  // Prints "<name>" (instance of <classname>, child of "<name>" <classname>).
+  // If a classloader has no name, it prints <unnamed> instead. The output
+  // for well known loaders (system/platform) is abbreviated.
+  static const char* describe_external(const oop loader);
 };