The approach is to desugar

  try { ... }
  catch ({x, y}) { ... }

into

  try { ... }
  catch (.catch) {
    let x = .catch.x;
    let y = .catch.y;
    ...
  }

using the PatternRewriter's normal facilities. This has the side benefit
of throwing the appropriate variable conflict errors for declarations
made inside the catch block.

No change is made to non-destructured cases, which will hopefully save
us some work if https://github.com/tc39/ecma262/issues/150 is adopted
in the spec.

There's one big problem with this patch, which is a lack of PreParser
support for the redeclaration errors. But it seems we're already lacking
good PreParser support for such errors, so I'm not sure that should
block this moving forward.

BUG=v8:811
LOG=y

Review URL: https://codereview.chromium.org/1417483014

Cr-Commit-Position: refs/heads/master@{#31797}