Driver: Make FailingCommand mandatory for generateCompilationDiagnostics

We currently use a null FailingCommand when generating crash reports
as an indication that the cause is FORCE_CLANG_DIAGNOSTICS_CRASH, the
environment variable that exists to test crash dumps. This means that
our tests don't actually cover real crashes at all, and adds a more
complicated code path that's only used in the tests.

Instead, we can have the driver synthesize that every command failed
and just call generateCompilationDiagnostics normally.