RFR: JDK-8241618 Fix unchecked warning for jdk.hotspot.agent

[Magnus Ihse Bursie]

With the recent fixes in JDK-8241310, JDK-8237746 and JDK-8241073, and 
the upcoming fixes to remove the deprecated nashorn and jdk.rmi, the JDK 
build is very close to producing no warnings when compiling the Java 
classes.

The one remaining sinner is jdk.hotspot.agent. Most of the warnings here 
are turned off, but unchecked and deprecation cannot be completely silenced.

Since the poor agent does not seem to receive much love nowadays, I took 
it upon myself to fix these warnings, so we can finally get a quiet build.

I started to address the unchecked warnings. Unfortunately, this was a 
much bigger task than I anticipated. I had to generify most of the 
module. On the plus side, the code is so much better now. And most of 
the changes were trivial, just tedious.

There are a few places were I'm not entirely happy with the current 
solution, and that at least merits some discussion.

I have resorted to @SuppressWarnings in four classes: ciMethodData, 
MethodData, TableModelComparator and VirtualBaseConstructor. All of them 
has in common that they are doing slightly fishy things with classes in 
collections. I'm not entirely sure they are bug-free, but this patch 
leaves the behavior untouched. I did some efforts to sort out the logic, 
but it turned out to be too hairy for me to fix, and it will probably 
require more substantial changes to the workings of the code.

To make the code valid, I have moved ConstMethod to extend Metadata 
instead of VMObject. My understanding is that this is benign (and likely 
intended), but I really need for someone who knows the code to confirm 
this. I have also added a FIXME to signal this. I'll remove the FIXME as 
soon as I get confirmation that this is OK.
(The reason for this is the following piece of code from Metadata.java: 
metadataConstructor.addMapping("ConstMethod", ConstMethod.class))

In ObjectListPanel, there is some code that screams "dead" with this 
change. I added a FIXME to point this out:
     for (Iterator<Oop> iter = elements.iterator(); iter.hasNext(); ) {
       if (iter.next() instanceof Array) {
         // FIXME: Does not seem possible to happen
         hasArrays = true;
         return;
       }
It seems that if you start pulling this thread, even more dead code will 
unravel, so I'm not so eager to touch this in the current patch. But I 
can remove the FIXME if you want.

My first iteration of this patch tried to generify the IntervalTree and 
related class hierarchy. However, this turned out to be impossible due 
to some weird usage in AnnotatedMemoryPanel, where there seemed to be 
confusion as to whether the tree stored Annotations or Addresses. I'm 
not entirely convinced the code is correct, it certainly looked and 
smelled very fishy. However, I reverted these changes since I could not 
get them to work due to this, and it was not needed for the goal of just 
getting rid of the warning.

Finally, I have done no testing apart from verifying that it builds. 
Please advice on suitable tests to run.

Bug: https://bugs.openjdk.java.net/browse/JDK-8241618
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8241618-fix-unchecked-warnings-for-agent/webrev.01

/Magnus