symbols: set tag_seen when assigning a type to symbols

* src/reader.h, src/reader.c (tag_seen): Move to...
* src/symtab.h, src/symtab.c: here.
(symbol_type_set): Set it to true.
* src/parse-gram.y: Don't.

src/reader.c

@@ -45,9 +45,6 @@ merger_list *merge_functions;
 /* Was %union seen?  */
 bool union_seen = false;
 
-/* Was a tag seen?  */
-bool tag_seen = false;
-
 /* Should rules have a default precedence?  */
 bool default_prec = true;