RFR [15] 8239243: Create index structures only if required

[Pavel Rappo]

Hello,

Please review the change for https://bugs.openjdk.java.net/browse/JDK-8239243:

  http://cr.openjdk.java.net/~prappo/8239243/webrev.00/

I noticed that the index structure, used in Index Page an others, is created
unconditionally (i.e. regardless of the presence of the "-noindex" command-line
option). Since populating that index doesn't seem to have any side-effects, this
will unnecessarily consume resources of documentation generation process when no
index is required. Thus, moving it under the relevant if-statement should be
safe and the right thing to do. I've also cleaned up the IndexBuilder class a bit.

In case you wonder, when generating the API docs for the JDK 15, it takes about
1 sec. (one second) to create that structure. The number of instances of Element
in that Map<Character, SortedSet<Element>> is roughly 50,000 (fifty thousand).

-Pavel