[Ethos-U] Replace ethos-u.constants with AllocateConst

[Lunderberg]

Previously, constants for ethos-u were tracked using a function attribute "ethos-u.constants". This predates the introduction of AllocateConst, and had comments indicating that it should be replaced with AllocateConst when possible.

To minimize impact to existing passes, this commit preserves the "ethos-u.constants" attribute during ethosu-specific lowering passes. The attribute is converted to AllocateConst at the end of the lower_ethosu pass, just prior to lowering with the usual TIR passes.

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[Lunderberg]

This change originated while debugging #14985, but ended up being unnecessary for it, and is therefore kept as a separate PR.

[ekalda]

Thanks for these improvements @Lunderberg! Looks like it needs a rebase and I'm not sure the changes to codegen_c_host.cc and split_host_device.cc were intentional for this this patch, but otherwise looks all good to me!

[Lunderberg]

Thank you for catching the extra changes in this PR. The codegen_c_host.cc changes definitely don't belong here, and are removed. The split_host_device.cc can probably be removed, as they're more closely related to #14985.