Unlike Clang, Swift's DeclContexts are not all Decls. However, I believe
each DeclContext that is /serialized/ will be either a decl, a
TranslationUnit, or a FuncExpr for a function with an actual declaration.
This might turn out to be wrong if (a) SIL needs proper DeclContexts for
variables in function bodies, or (b) we need to serialize anonymous
closure default arguments.
Along with an extension of the ConstructorDecl placeholder code, this allows
us to round-trip empty structs.
Swift SVN r5532
