Dropping the requirement for C++-only bootstrapping

[etcwilde (Evan Wilde)]

@3405691582

it might be nice to consider keeping a 6.2 branch (or a branch of whatever might be considered the Last Release in the future) open strictly for bootstrap related issues that might arise when bringing Swift to new platforms in the future,

Not a long shot. This is exactly the plan. I don't have a branch name yet since we haven't had to bring one up, but it'll probably be something like 6.2/bootstrap.