Destroy the diagnostic client first in ~DiagnosticEngine

Add a comment and a test to ~DiagnosticEngine about the ordering
requirements on the teardown of DiagnosticConsumer. This could also be
accomplished by rearranging the fields of ~DiagnosticEngine, but I felt
that this was a better, more explicit solution.

This fixes PR21911, an issue that occurred after the unique_ptr
migration in r222193.