[code-reflection] RFR: Further cleanup of `CodeModelToAST` and regularize support for varargs instance creation expressions

[Maurizio Cimadamore]

This PR makes the code in `CodeModelToAST` that deal with instance creation expression (`NewOp`) a bit closer to the code that deals with method calls (`InvokeOp`). I've added a method, called `constructorTypeToSymbol` which takes the constructor type of a `NewOp` and looks up the constructor symbol with that type.

Then I moved the logic that sets `varargElements` on the created `JCMethodInvocation` nodes into a separate, reusable routine. This is now called also for varargs instance creation expression.

Of course, to do that I had to add support for a new varargs attribute in the `NewOp` class -- which closely follows the one used by `InvokeOp`. This means that in `ReflectMethod` we can now reify in the model as to whether a constructor call was varargs or not.

I've also added some arg count validation in `NewOp`, very similar to the one that appears in `InvokeOp`.

-------------

Commit messages:
 - Add validation
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/393/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=393&range=00
  Stats: 194 lines in 7 files changed: 151 ins; 14 del; 29 mod
  Patch: https://git.openjdk.org/babylon/pull/393.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/393/head:pull/393

PR: https://git.openjdk.org/babylon/pull/393