The language server and unit tests pass in an empty output directory
to signal that no C++ files should be generated. As these
generation steps include some validations, they should also be
included in language server and unit test compilation runs.

This CL introduces a "dry run" flag on the ImplementationVisitor.
Additionaly, the implementation visitor wraps the file writing
functionality. In case of a dry-run, file writing becomes a no-op.

R=sigurds@chromium.org

Bug: v8:7793
Change-Id: Id699fdf0b35311ddd3c1f5419baa0237b40ddce4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617244Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61599}