Function bodies are skipped during typechecking when one of the
-experimental-skip-*-function-bodies flags is passed to the frontend. This was
implemented by setting the "body kind" of an `AbstractFunctionDecl` during decl
checking in `TypeCheckDeclPrimary`. This approach had a couple of issues:
- It is incompatible with skipping function bodies during lazy typechecking,
since the skipping is only evaluated during a phase of eager typechecking.
- It prevents skipped function bodies from being parsed on-demand ("skipped" is
a state that is distinct from "parsed", when they ought to be orthogonal).
This needlessly prevented complete module interfaces from being emitted with
-experimental-skip-all-function-bodies.
Storing the skipped status of a function separately from body kind and
requestifying the determination of whether to skip a function solves these
problems.
Resolves rdar://116020403
