Their definition is fully visible to clients to be copied into them, and there isn't necessarily
a symbol for the property descriptor in the defining module, so it isn't necessary or desirable to
try to use a property descriptor with them. Trying to reference the descriptor leads to missing
symbol errors at load time when trying to use keypaths with older versions of the defining dylib,
which goes against the purpose of `@_alwaysEmitIntoClient` meaning "no ABI liabilities for the
defining module". Fixes rdar://94049160.
With a few modifications:
- Changing computed get-only properties into 'let's breaks resilience in other ways we need to fix independent of key path resilience
- Fix the tests not to ask for impossible lib-before--client-after compatibility for changes that add API
