-Fast bot upgrades

-Fast bot upgrades

  - GCC -> Clang;
  - Add a Perf bot.

This -Fast mode is a stand-in for "how clients should/do really release code".  Today everyone uses Clang, so we should probably switch this bot too.

I also got interested in how it compares in performance to our ordinary Release build, so I've added a Perf bot.

I did try turning on link-time optimization (much slower build, somewhat faster binary), but that failed at the link step.  Going to save that for later.

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Ubuntu-Clang-x86_64-Release-Fast,Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-Fast,Perf-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-Fast

Change-Id: I58e6109f8a89799d80e769316902549131b619cf
Reviewed-on: https://skia-review.googlesource.com/6595
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>