[clang] [Clang] Allow all address spaces to be converted to the default (PR #112248)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> So overall I'm leaning against doing it as is; guarding it under some flavour of `allow-relaxed-address-space-handling` or something that sounds better but is still noisy enough to make it clear there's danger seems preferable, at least to me.

A language option is reasonable if we don't want default behavior, though it does add a bit of magic to what is otherwise straightfoward on all the other standard targets.

https://github.com/llvm/llvm-project/pull/112248