Add and use a clang_linux asset.

This gives us a consistent Clang toolchain on Linux bots,
most importantly for *SAN bots.  It's ~300MB unpacked.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2294353002

Review-Url: https://codereview.chromium.org/2294353002
16 files changed