In the compile-clang task we provide the following expansions
    CLANG_C_VERSION: -DCLANG_C_VERSION=6.0
    CLANG_CXX_VERSION: -DCLANG_CXX_VERSION=6.0

but don't evaluate them in the configure wiredtiger task which means our compile tasks fall back on the default version of clang instead of the version we want to test. To fix this we need to update configure wiredtiger to accept these variables in the same manner GCC_C_VERSION and GCC_CXX_VERSION are currently implemented.

We intentionally don't use the mongotoolchain for these tests, so we need to update the variants on this branch and older branches as well as adding a comment explaining why we don't want the mongotoolchain.