We should add a clang 3.9 builder to our Evergreen CI loop, to test against a newer version of clang than what we currently test against (which is 3.8, what we run on the ASAN and UBSAN builders).

Although the host types we currently use in Evergreen don't have a system-installed version of clang 3.9 available, we can use clang 3.9 from /opt/mongodbtoolchain/v2. If related ticket BUILD-3005 is completed before this work, we should update the description for this ticket to refer to clang 4.0.