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This is a reland of be8138a4
The issues with the previous attempt were addressed in the latest
Clang roll (#553415).

Original change's description:
> win: Link with lld instead of MSVC's link.exe by default
>
> lld is LLVM's linker. It produces PE/COFF and PDB files just like
> link.exe, but it's significantly faster and it can also handle LLVM's
> internal representation, which will enable us to do link-time
> optimization and control-flow integraty checks with Clang.
>
> While lld is much faster at linking, it doesn't support incremental
> links, meaning builds that only touch a few files and re-link a large
> executable may become slower.
>
> This is the first attempt at switching everything over, with the
> purpose of gathering data and finding unknown unknowns. It's likely
> temporary until something breaks.
>
> is_win_fastlink is implicitly ignored when using lld, as lld without
> fastlink is faster than link.exe with it.
>
> Also switch the CrWinClangLLD bots on chromium.clang to use MSVC's
> link.exe to make sure that configuration keeps working.
>
> Bug: 792131
> Change-Id: I0f115a78c33d69eadbd480f75c2a5d636e86483d
> Reviewed-on: https://chromium-review.googlesource.com/983632
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#551367}

Bug: 792131
Change-Id: I3b76cd015ef18bb3e2ac7a3efa4352c4f04e560b
Reviewed-on: https://chromium-review.googlesource.com/1028374Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553776}