Remove DT_Failed mode

At the moment if -dynamic-too fails then we rerun the whole pipeline as if we were just in -dynamic mode. I argue this is a misfeature and we should remove the so-called DT_Failed mode.

In what situations do we fall back to DT_Failed?

1. If the dyn_hi file corresponding to a hi file is missing completely.
2. If the interface hash of dyn_hi doesn't match the interface hash of hi.

What happens in DT_Failed mode?

• The whole compiler pipeline is rerun as if the user had just passed -dynamic.
• Therefore dyn_hi/dyn_o files are used which don't agree with the hi/o files. (As evidenced by dynamicToo001 test).
• This is very confusing as now a single compiler invocation has produced further hi/dyn_hi files which are different to each other.

Why should we remove it?

• In --make mode, which is predominately used DT_Failed does not work (#19782 (closed)), there can't be users relying on this functionality.
• In -c mode, the recovery doesn't fix the root issue, which is the dyn_hi and hi files are mismatched. We should instead produce an error and pass responsibility to the build system using -c to ensure that the prerequisites for -dynamic-too (dyn_hi/hi) files are there before we start compiling.
• It is a misfeature to support use cases like dynamicToo001 which allow you to mix different versions of dynamic/non-dynamic interface files. It's more likely to lead to subtle bugs in your resulting programs where out-dated build products are used rather than a deliberate choice.
• In practice, people are usually compiling with -dynamic-too rather than separately with -dynamic and -static, so the build products always match and DT_Failed is only entered due to compiler bugs (see !6583 (closed))

What should we do instead?

• In --make mode, for home packages check during recompilation checking that dyn_hi and hi are both present and agree, recompile the modules if they do not.
• For package modules, when loading the interface check that dyn_hi and hi are there and that they agree but fail with an error message if they are not.
• In --oneshot mode, fail with an error message if the right files aren't already there.

cc @hsyl20 @int-e