`discard self` requires knowledge of the internal layout of the type in order to clean
up its fields while bypassing its public `deinit`. Inlinable code can get copied into
and run from outside of the defining module, so this is impossible to implement.
Fixes rdar://160815058.
One can still in resilient frameworks have noncopyable frozen types.
This means that one cannot make a noncopyable:
1. Full resilient public type.
2. @usableFromInline type.
NOTE: One can still use a frozen noncopyable type as a usableFromInline class
field. I validated in the attached tests that we get the correct code
generation.
I also eliminated a small bug in TypeCheckDeclPrimary where we weren't using a
requestified attr check and instead were checking directly.
rdar://111125845
SE-390 concluded with choosing the keyword discard rather than forget for
the statement that disables the deinit of a noncopyable type. This commit
adds parsing support for `discard self` and adds a deprecation warning for
`_forget self`.
rdar://108859077
