The other side of #17404. Since we don't want to generate up front key path metadata for properties/subscripts with no withheld implementation details, the client should generate a key path component that can be used to represent a key path component based on its public interface.
The SILGen testsuite consists of valid Swift code covering most language
features. We use these tests to verify that no unknown nodes are in the
file's libSyntax tree. That way we will (hopefully) catch any future
changes or additions to the language which are not implemented in
libSyntax.
A public subscript might have generic indexes that aren't unconditionally Hashable, or might use indexes that are retroactively made Hashable, so the property descriptor on the implementer's side can't always resiliently provide this information to the final instantiated KeyPath.
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
This way we'll link against the key path component the other module provides instead of making fragile assumptions about its current implementation. Since external keypath lowering isn't fully implemented elsewhere in the compiler, this is enabled behind a staging flag.
external keypath staging
