Remove DT_Failed statewip/dyn-too-fixes

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), 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)

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.

Closes #19782 #20446 #9176 #13616

1 files changed, 5 insertions, 0 deletions

diff --git a/testsuite/tests/driver/dynamicToo/dynamicTooMake/README.md b/testsuite/tests/driver/dynamicToo/dynamicTooMake/README.md
new file mode 100644
index 0000000000..5b9ecd01f1
--- /dev/null
+++ b/testsuite/tests/driver/dynamicToo/dynamicTooMake/README.md
@@ -0,0 +1,5 @@
+Reproducer for broken implementation of DT_Failed
+
+```
+./run
+```