Add support in JVM TI, JDI, and Instrumentation for hidden classes

From JVM's point of view, a hidden class is a normal class except the following:

   1. A hidden class has no initiating class loader and is not registered in any dictionary.
   2. A hidden class has a name containing an illegal character
      `Class::getName` returns `p.Foo/0x1234` whereas `GetClassSignature` returns "Lp/Foo.0x1234;".
   3. A hidden class is not modifiable, i.e. cannot be redefined or retransformed.
      JVM TI (IsModifableClass)[https://download.java.net/java/early_access/jdk15/docs/specs/jvmti.html#IsModifiableClass] returns false on a hidden class and VM anonymous class.

VM anonymous class vs Hidden class

   - VM anonymous class is not registered in any dictionary and is also not modifiable.
   - There is no specification for VM anonymous class and hence never specified
     it has or does not have an initiating loader.
   - GetClassSignature returns "Lp/Foo/0x1234;" on a VM anonymous class with no illegal character
     whereas GetClassSignature returns "Lp/Foo.0x1234;" for a hidden class.
     VM anonymous class might be incorrectly interpreted as a normal class and
     `p.Foo` as the package name.

The proposal is that hidden classes are deemed as "loaded" classes, i.e. essentially all derived classes. Hidden classes are not "treated" specially.