Add sanitizer-x86_64-linux-fast bot.

Add a new sanitizer bot. The host compiler is clang fixed at a specific
revision; it will not be rebuilt unless the revision number is changed in the
bot config. The host compiler does incremental builds of (A,M)San-instrumented
LLVM/clang, which are then tested with check-llvm, check-clang.

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@224445 91177308-0d34-0410-b5e6-96231b3b80d8