Look up type again if accessing incomplete structs.

If a struct was forward-declared, then completed later, we previously
didn't always allow accessing fields on that struct. It depended on
whether typedefs were involved and what order the declarations appeared
in. This commit ensures that we always find the right type declaration,
if it's in scope at the time of use.

Author: jameysharp

src/Language/Rust/Corrode/C.md

@@ -1825,6 +1825,11 @@ interpretExpr _ expr@(CMember obj ident deref node) = do
     obj' <- interpretExpr True $ if deref then CUnary CIndOp obj node else obj
     fields <- case resultType obj' of
         IsStruct _ fields -> return fields
+        IsIncomplete tyIdent -> do
+            (_, struct) <- getIdent (StructIdent tyIdent)
+            case struct of
+                Just (_, IsStruct _ fields) -> return fields
+                _ -> badSource expr "member access of incomplete type"
         _ -> badSource expr "member access of non-struct"
     let field = identToString ident
     ty <- case lookup field fields of