Support deprecated annotation on messages, fields, enums, enum values.

descriptor.proto defines a deprecated bool on message, enum, enum value, field; the generated things should get the Swift annotations also.

note: Extensions should honor it since they are a field.

Start with descriptor.proto and trace the generator entry point that handles deprecated fields on messages, enums, enum values, and fields. Check how extensions are represented as fields, then verify that generated Swift declarations receive the corresponding annotations for every listed element and for extensions.

Written by the indexing model from the issue text.