SaveFlags previously worked by re-setting the flags using the command
line. Unfortunately, this could reset flags being used by concurrent
processes, which would cause TSAN issues.

Now, SaveFlags stores a copy of the state of all flags on creation, and
only resets changed flags in its destructor. It does this by (ab)using
the flag-definitions.h pseudo-header, adding a new mode to that header
which applies an includer-defined macro to each flag definition.

Change-Id: I4c156ecb36b4b7c05402138088266465d31e33b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1530809Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60350}