mb: Add src-side changes for a new "win_clang" bot.
mb: Add src-side changes for a new "win_clang" bot.

I want to use this bot as the default-on win/clang trybot, so that if I need
to modify it (change it to release, or make it use LLD, or make it build fewer
targets then "all"), I don't have to modify our existing FYI waterfall bots.

BUG=588518

Review URL: https://codereview.chromium.org/1838083003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383909}

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