Historically we've disabled these assertions on a number of platforms with the
goal of speeding up CI. Now, though, having migrated to GitHub actions, CI is
already pretty fast, and these debug assertions do bring us some value.

This does leave in some debug assertions that are performance-related: macOS
currently hovers at just under 2 hours.

There are also some other builders which have debug and LLVM assertions
disabled:

llvm-8, PR builder:

In one view, this builder tests our support for older LLVMs. But in reality, a
lot of our tests already disable themselves on older LLVMs, and I think our
general stance is that we really only support the in-tree LLVM. Plus, we really
want CI times on this builder to be really low, as it's run on *every* PR --
that's a lot of CI time.

test-various:

This disables debug asserts still -- as noted in the Dockerfile, we test code
size, and we need debug asserts off for that to work well.