Reland of [gn] Switch win clang bot to gn (patchset #1 id:1 of https://codereview.chromium.org/2311853002/ )

Reason for revert:
Trying reland with plugin switched off.

Original issue's description:
> Revert of [gn] Switch win clang bot to gn (patchset #1 id:1 of https://codereview.chromium.org/2310863002/ )
>
> Reason for revert:
> Fails compilation:
> https://build.chromium.org/p/client.v8/builders/V8%20Win64%20-%20clang
>
> Original issue's description:
> > [gn] Switch win clang bot to gn
> >
> > BUG=chromium:474921
> > NOTRY=true
> >
> > Committed: https://crrev.com/1740587772bbd1ffb7b79a90deb16519e8d47588
> > Cr-Commit-Position: refs/heads/master@{#39156}
>
> TBR=jochen@chromium.org,vogelheim@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=chromium:474921
>
> Committed: https://crrev.com/49e13ea6eab4158915a75b95ccc9f01892924d9e
> Cr-Commit-Position: refs/heads/master@{#39158}

TBR=jochen@chromium.org,vogelheim@chromium.org
NOTRY=true
BUG=chromium:474921,chromium:644096

Review-Url: https://codereview.chromium.org/2312093002
Cr-Commit-Position: refs/heads/master@{#39201}