The most important point of IWYU (include-what-you-use) is that each
header includes everything it is using, so that whoever includes that
header does not need to additionally include other things.
This CL adds a script which generates files to automatically check this.
It is automatically invoked during "gclient runhooks" if the
"check_v8_header_includes" variable is set. This script generates a
number of .cc files in the "check-header-includes" directory, together
with a "sources.gni" file which lists all the generated cc files. Each
file includes one header.
If additionally the gn args "v8_check_header_includes" is set, this gni
file is included, and all the generated CC files will be compiled. This
will detect violations of the aforementioned IWYU rule.

R=titzer@chromium.org, machenbach@chromium.org

Bug: v8:7754, v8:7965
Change-Id: Id1cf256507052c3a9ea82f8c80ea1c0385457e31
Reviewed-on: https://chromium-review.googlesource.com/1145199
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54590}