Add loadDynamicallyReplacedFunctionDecl
To support lazy resolution of the cross-referenced function in a serialized @_dynamicReplacement(for: ...) attribute, add a utility to the LazyMemberLoader and plumb it through. This is a more general utility than the current resolver, which relies on the type checker to strip the attribute off of VarDecls and fan it back out onto accessors, which means serialization has only ever seen AbstractFunctionDecls.
