Don't evaluate discriminator value constants when parsing.

Remove disr_val from ast::variant_ and always use ty::variant_info
when the value is needed.  Move what was done during parsing into
other passes, primary typeck.rs.  This move also correctly type checks
the disr. value expression; thus, fixing rustc --pretty=typed when
disr. values are used.

src/comp/syntax/ast.rs

@@ -409,7 +409,7 @@ type native_mod =
 type variant_arg = {ty: @ty, id: node_id};
 
 type variant_ = {name: ident, args: [variant_arg], id: node_id,
-                disr_val: int, disr_expr: option::t<@expr>};
+                 disr_expr: option::t<@expr>};
 
 type variant = spanned<variant_>;