Introduce @lifetime attribute to specify lifetime dependence on function declarations

[meg-gupta]

This implements an alternate way to specify lifetime dependence as described here - https://github.com/swiftlang/swift-evolution/blob/9ba7a574d1557eefb4bc3cce1d07efee51861f21/proposals/NNNN-lifetime-dependency.md#lifetime-annotation

rdar://134975439

TODO: Support lifetime dependence on parameter and self targets

[atrick]

This looks good for now. Thanks!

One thing we should consider going forward: we may want separate attributes for scoped vs. inherited dependencies. In that world @lifetime would always be an inherited lifetime dependency. A separate attribute would indicate whether a parameter is "borrowed". Let's say @borrow.

Why a separate attribute?

• Conceptually, we want to separate the concept of a projection vs. a lifetime dependency. Then we don't need to talk about two different kinds of dependencies.

• @borrow is important for any "value projection" even when the projected value is not ~Escapable. We'll need for ~Copyable projections so that we can find the correct root for a lifetime dependency. It will also be useful for Copyable projections where we don't want copies to be automatically generated for the projected value.

• Inferring the kind of dependence from the type may get confusing for conditionally ~Escapable types. For example, I think it should be illegal to have a (copied) dependency from a conditionally ~Escapable value to an unconditionally ~Escapable value.

[meg-gupta]

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[meg-gupta]

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[meg-gupta]

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