NCGenerics: avoid feature-guarding in some cases
With the generalization of Optional to support noncopyable types, our
feature-guarding in swiftinterface files would double-print functions
that simply refer to the Optional type.
Since NoncopyableGenerics is a suppressible feature, by default
a second version of Optional and UnsafePointer are emitted into
swiftinterface files, where the ~Copyable generalization is stripped
away.
We can rely on that to avoid double-printing the function, if the types
substituted for the generic parameters are all Copyable.
We need a bit more checking for when
`@_disallowFeatureSuppression(NoncopyableGenerics)` is used, since this
trick relies on there always being a definition of the type we refer to,
whether the feature is enabled or not.
resolves rdar://127389991
