Put all XSAN bots on the same DEPS-sourced build of Clang.

Put all XSAN bots on the same DEPS-sourced build of Clang.

This adds LLVM, Clang, and compiler-rt (*san tools) to DEPS,
then uses them from xsan_build, building them if needed.

This is similar to how the CMake bots bootstrap CMake if needed.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1693733003

Review URL: https://codereview.chromium.org/1693733003