Add a new lint UNCONSTRUCTABLE_PUB_STRUCT to detect unconstructable public structs

[mu001999]

Add a new lint UNCONSTRUCTABLE_PUB_STRUCT to detect unconstructable public structs, based on the following observations:

1. A public struct with private field(s) cannot be directly constructed from external crates.
2. Associated functions with a receiver require an already constructed value of type Self.
3. Therefore, public structs with private fields and their associated functions that take a receiver can be included in the local crate's dead code analysis.
4. If a public struct with private fields cannot be constructed in any reachable code path, it could be considered dead. Otherwise, liveness can be propagated to the struct and its associated functions with a receiver during the dead code analysis.

[rustbot]

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[juntyr]

Would the lint fire on token structs that are public, have private fields, have no public constructor method, but expose a limited number of pre-constructed objects, e.g. through a static that contains an optional token?

[mu001999]

a static that contains an optional token

won't fire like private types used in such places

[davidtwco]

Nominating for t-lang to decide whether we want this lint, then I'll review the implementation.

Also, s/unconstructible/unconstructable.