style: clarify the use of symbol_lists' locations

symbol_list features a 'location' and a 'sym_loc' member.  The former
is expected to be set only for symbol_lists that denote a symbol (not
a type name), and the latter should only denote the location of the
symbol/type name.  Yet both are set, and the name "location" is too
unprecise.

* src/symlist.h, src/symlist.c (symbol_list::location): Rename as
rhs_loc for clarity.  Move it to the "section" of data valid only
for rules.
* src/reader.c, src/scan-code.l: Adjust.

src/symlist.h

@@ -50,7 +50,6 @@ typedef struct symbol_list
      */
     semantic_type *sem_type;
   } content;
-  location location;
 
   /* Named reference. */
   named_ref *named_ref;
@@ -74,6 +73,9 @@ typedef struct symbol_list
   | when several RHSs are bound to a single lhs via "|").         |
   `--------------------------------------------------------------*/
 
+  /* Location of the RHS. */
+  location rhs_loc;
+
   /* Precedence/associativity.  */
   symbol *ruleprec;