The compiler treats version tuples that are all zeros as empty, or the same as
not having a version. Diagnose attempts to specify all-zeroes versions in
attributes and availability queries to prevent surprising behavior.
Resolves rdar://124661151
Previously, typechecking and SILGen would treat a function body as fragile as long as the declaration had a `@backDeployed` attribute, regardless of the platform specified by the attribute. This was overly conservative since back deployed functions are only emitted into the client on specific platforms. Now a `@backDeployed` function can reference non-`public` declarations on the platforms it is resilient on:
```
@backDeployed(before: iOS 15)
public func foo() {
#if os(iOS)
// Fragile; this code may be emitted into the client.
#else
// Resilient; this code won't ever be exposed to clients.
#endif
}
```
Resolves rdar://105298520
