[torque] move Antlr to separate BUILD.gn file

Change-Id: I59517bd8e2176b5ec64401d5418d7a4ecb1bcc52
Reviewed-on: https://chromium-review.googlesource.com/1033733
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52910}