This is a reland of d8f8a7e2
Change compared to last reland:
- Do not check for d8 flag contradictions in the presence of --fuzzing
- Allow identical re-declaration of --cache=*

Original change's description:
> Reland "[flags] warn about contradictory flags"
>
> This is a reland of b8f91666
> Difference to previous CL: Additional functionality to specify
> incompatible flags based on GN variables and extra-flags, used
> to fix the issues that came up on the waterfall.
>
> This also changes the rules regarding repeated flags: While
> explicitly repeated flags are allowed for boolean values as long
> as they are identical, repeated flags or explicit flags in the
> presence of an active implication are disallowed for non-boolean
> flags. The latter simplifies specifying conflict rules in
> variants.py. Otherwise a rule like
>
> INCOMPATIBLE_FLAGS_PER_EXTRA_FLAG = {
>   "--gc-interval=*": ["--gc-interval=*"],
> }
>
> wouldn't work because specifying the same GC interval twice
> wouldn't actually count as a conflict. This was an issue with
> test/mjsunit/wasm/gc-buffer.js, which specifies
> --gc-interval=500 exactly like the extra flag by the stress bot.
>
> Also, this now expands contradictory flags checking to d8 flags
> for consistency.
>
> Original change's description:
> > [flags] warn about contradictory flags
> >
> > Design Doc: https://docs.google.com/document/d/1lkvu8crkK7Ei39qjkPCFijpNyxWXsOktG9GB-7K34jM/
> >
> > Bug: v8:10577
> > Change-Id: Ib9cfdffa401c48c895bf31caed5ee03545beddab
> > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2154792
> > Reviewed-by: Clemens Backes <clemensb@chromium.org>
> > Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> > Reviewed-by: Georg Neis <neis@chromium.org>
> > Reviewed-by: Tamer Tas <tmrts@chromium.org>
> > Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#68168}
>
> Bug: v8:10577
> Change-Id: I268e590ee18a535b13dee14eeb15ddd0a9ee8341
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235115
> Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> Reviewed-by: Tamer Tas <tmrts@chromium.org>
> Reviewed-by: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#68989}

Bug: v8:10577
Change-Id: I31d2794d4f9ff630f3444210100c64d67d881276
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2339464
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69339}