win: Unconditionally download clang during runhooks

This adds an additional ~50MB download to runhooks every few weeks.
The motivation for doing this is two-fold:

1. It makes it easier for people to try out clang on Windows
2. libyuv currently uses relatively unoptimized C-only functions
   on 64-bit Windows due to cl not supporting inline assembly.
   clang-cl does, and building libyuv with clang-cl speeds up
   some libyuv conversion functions to take 0.4ms instead of 2ms
   per frame, so we want to try and build just libyuv with clang
   even in MSVC builds.

BUG=692600,82385

Review-Url: https://codereview.chromium.org/2702113005
Cr-Commit-Position: refs/heads/master@{#451827}
1 file changed