Fix type parsing

the check on incomplete struct/union/enum types was too early,
disallowing mixed specifiers and qualifiers.  Simply rely on
the size (->c) field for that.  See testcases.

tests/tests2/61_undefined_enum.expect

@@ -1 +1 @@
-61_undefined_enum.c:1: error: unknown struct/union/enum
+61_undefined_enum.c:1: error: unknown type size

tests/tests2/81_types.c

@@ -0,0 +1,9 @@
+/* The following are all valid decls, even though some subtypes
+   are incomplete.  */
+enum E *e;
+const enum E *e1;
+enum E const *e2;
+struct S *s;
+const struct S *s1;
+struct S const *s2;
+int main () { return 0; }