More in general, the managed renderer flow does things more sensibly
in case an exception is thrown:
* it will not remove any file
* it will drop already written files from the registry, so that codegen
won't be skipped for those files during the next run
This was causing hardly debuggable errors because names are transformed
to underscored lowercase names in the dbscheme and back to camelcase
for trap emission classes, which is not a noop in case uppercase
acronyms (like SIL or ABI) are in the name.
This makes the error be surfaced early with a helpful message.
This is a developer QoL improvement, where running codegen will skip
writing (and especially formatting) any files that were not changed.
**Why?** While code generation in itself was pretty much instant, QL
formatting of generated code was starting to take a long time. This made
unconditionally running codegen quite annoying, for example before each
test run as part of an IDE workflow or as part of the pre-commit hook.
**How?** This was not completely straightforward as we could not work
with the contents of the file prior to code generation as that was
already post-processed by the QL formatting, so we had no chance of
comparing the output of template rendering with that. We therefore store
the hashes of the files _prior_ to QL formatting in a checked-in file
(`swift/ql/.generated.list`). We can therefore load those hashes at
the beginning of code generation, use them to compare the template
rendering output and update them in this special registry file.
**What else?** We also extend this mechanism to detect accidental
modification of generated files in a more robust way. Before this patch,
we were doing it with a rough regexp based heuristic. Now, we just store
the hashes of the files _after_ QL formatting in the same checked file,
so we can check that and stop generation if a generated file was
modified, or a stub was modified without removing the `// generated`
header.
In those kinds of tests the results may have different final classes
that are not necessarily visible (or tested) solely through the string
representation. For better testing and reading of expected results,
`getQlPrimaryClasses` is added in these cases.
