rdar://64344067 (Architecture specialization)

[neonichu]

This implements a "lite" version of architecture specialization, piggy-backing on superimposed parameters. The lite part is that we do not actually build an accurate set of architectures per client but instead will build the superset of all architectures required by any client, plus the default set of architectures inherent to the target. This means we might be overbuilding in certain cases, but we should no longer have issues with arches that are required by client targets missing from specialized targets.

This should be functional in basic terms, but has no tests yet and will break any tests involving specialization which closely monitor overrides due to the extra ARCHES override added here. I haven't yet decided whether I want to try to prune overrides that are superficial or update the affected tests.

[neonichu]

@swift-ci please test

[neonichu]

I'm also seeing a bunch of warnings like "warning: Could not determine settings for X when computing Swift AST paths for Y" since that computation is not using the real settings but settingsForProductReferenceTarget. I guess settingsForProductReferenceTarget should be updated to splat over ARCHES as well?

[neonichu]

The final merging logic needs to actually take platform specialization into account, right now we would be merging arches across platforms incorrectly.

I think that also means the merging logic might not necessarily work right today for multiple specialized targets.