Move AST related utilities to compiler/translator/tree_util.

BUG=angleproject:2409
TEST=angle_unittests

Change-Id: I7567c2f6f2710292029263257c7ac26e2a144ac8
Reviewed-on: https://chromium-review.googlesource.com/966032Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>