Add an option to not emit unionsplit fallback blocks (#43923)
When union splitting, we currently emit a fallback block that prints
`fatal error in type inference (type bound)`. This has always been an oddity,
because there are plenty of other places in the compiler that we
rely on the correctness of inference, but it has caught a number of
issues over the years and we do still have a few issues (e.g. #43064)
that show this error. Nevertheless, the occurrence of issues like
this has become less frequent, so it might be time to turn it off soon.
At the same time, we have downstream users of the compiler infrastructure
that get confused by having extra `throw` calls inserted into functions
that (post-#43852) were inferred as `:nothrow`.
Here we add an optimization param (defaulted to on) to determine whether
or not to insert the unionsplit fallback block. Because of the conservative
default, we can decide later what the correct default is (maybe turn it on
in `debug` mode?), while letting downstream consumers play with the setting
for now to see if any issues crop up.
