Currently, it is possible to make the Torque compiler emit uncompilable
C++ code if a 'generates' clause is missing. Rather than making the
developer spelunk through generated code to find out what went wrong,
we can catch this error earlier and print a useful message.

Bug: v8:7793
Change-Id: I49fabc1d39f398bf322523901941494b8dab0506
Reviewed-on: https://chromium-review.googlesource.com/c/1477964Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#59744}