Extend the effect of --freeze-flags-after-init to also protect updates
of individual flags instead of only the API.
For this, we wrap each flag in a {FlagValue} class which implicitly
converts to the value of the flag. Some cases still require the explicit
{value()} accessor though. That accessor is {constexpr}, in contrast to
the implicit conversion, because otherwise clang emits a lot of warnings
about dead code within "if (FLAG...)" scopes.

R=cbruni@chromium.org

Bug: v8:12887
Change-Id: I87d3457e49ceb317d34d6a21cf09c520d4171eb5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3683321Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80938}