Require clang in stage 1 when enabling stage 2 / test-suite

Both the test-suite and stage 2 need clang, so check early that it is
built as part of stage 1.

I'm not 100% certain on the former, since nowadays the test-suite
contains some bitcode tests which might be able to run without clang,
but it's unlikely that we'll ever be interested in such a scenario.

Change-Id: I6668557d9c2ef8cbad8a309a3c73c690d24533d1
2 files changed