ManualOwnership: permit use in production builds
This feature is just to enable a new set of diagnostics
that one can opt-into with `@diagnose` (nee `@warn`) or
with `-Wwarning`/`-Werror`.
There's no material impact on programs; it's more of a SIL pass
that you can opt into turning on that will emit diagnostics.
The current implementation does slightly change the emitted
code, so it's not just on-always until it's fully tested.
For example, it does change how no-implicit-copies enforcement
happens for `borrowing` parameters of functions, making it
effectively optional based on your preference for seeing warnings
about `SemanticCopies`. I think that's a better direction to head
towards anyway.
rdar://175897573
