Currently not all types are visited in canSerialize* calls, sometimes
resulting in an internal type getting @usableFromInline, which is
incorrect.
For example, for `let q = P() as? Q`, where Q is an internal class
inherting a public class P, Q is not visited in the canSerialize*
checks, thus resulting in `@usableFromInline class Q`; this is not
the intended behavior in the conservative mode used by PackageCMO
as it modifies AST.
To properly fix, instruction visitor needs to be refactored to do
both the "canSerialize" check (that visits all types) and serialize
or update visibility (modify AST in non-conservative modes).
This PR provides a short-term fix that prevents modifying AST, and
also ensures that the generated interfaces with PackageCMO flags
are not affected by the optimization or contain modified AST.
rdar://130292190
