backend=auto (now the default if backend is omitted) means to let the compiler
pick whatever backend it wants as the default. This is important for platforms
where we don't yet have a self-hosted backend, such as loongarch64.
Also purge a bunch of redundant target=native.
This is all of the expected 0.14.0 progress on #21530, which can now be
postponed once this commit is merged.
This required rewriting the (un)wrap operations since the original
implementations were extremely buggy.
Also adds an easy way to retrigger Sema OPV bugs so that I don't have to
keep updating #22419 all the time.
Maybe I'm just being pedantic here (most likely) but I don't like how we're
just telling the user here how to "suppress this error" by "assigning the value to '_'".
I think it's better if we use the word "discard" here which I think is
the official terminology and also tells the user what it actually means
to "assign the value to '_'".
Also, using the value would also be a way to "suppress the error".
It is just one of the two options: discard or use.
