Since we switched to using the scope stack instead of the decl state
stack to hold information about the containing definition in #3460 , we
can now pop the decl state at the end of the declaration instead of the
end of the body of the definition.
Continuing along with #3070. Note this is just a file rename, with BUILD
edits; every file previously in semantics/ should show as moved (except
maybe BUILDs, which split).