Enable goldens for cpp

This makes it easier to review and work on template generators. To see
changes due to generator change, developers can:

1. make their changes
2. run `mojo/golden/generate.py` (< 1s)
3. run `git diff`

This will show the changes stemming from template changes.

Change-Id: I901d0321d8b5e1dccc7d135ac04434428aff05b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6350159
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Fred Shih <ffred@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434420}