RFR: JDK-8238648 Rename and simplify Utils.WeakSoftHashMap

[Jonathan Gibbons]

Please review a 1-file cleanup to rename and simplify Utils.WeakSoftHashMap.

Utils.WeakSoftHashMap sounds like a general collection but is 
specifically an impl of Map<Element,CommentHelper>.

Furthermore, although it declares that it implements 
Map<Element,CommentHelper>, it is always used explicitly via its own 
class name, and never as a Map. This means that we can determine the 
methods that are required (get, put, remove, computeIfAbsent) and delete 
the others.

Finally, it is "just" a memory sensitive cache for CommentHelper 
objects, and so a better name for the class is just CommentHelperCache.

That all being said, there is one small but notable change. Previously, 
the code used a WeakHashMap<Element, ...>, so that the cache would not 
hold on unnecessarily to the keys. But the reality is that javac never 
drops references to elements in the javac symbol tables, and javadoc has 
many other Map<Element, ?> and so there is really no point in this cache 
using a WeakHashMap. It is therefore changes to a plain simple HashMap.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8238648
Webrev: http://cr.openjdk.java.net/~jjg/8238648/webrev/index.html