Revert "Make [] and ~[] both construct ivecs"

This reverts commit 60e1cead9b.

The check-fast driver can't work with this commit. Need to wait until main
taks ivecs

src/comp/syntax/parse/parser.rs

@@ -803,7 +803,7 @@ fn parse_bottom_expr(p: &parser) -> @ast::expr {
         let es =
             parse_seq_to_end(token::RBRACKET, some(token::COMMA), parse_expr,
                              p);
-        ex = ast::expr_vec(es, mut, ast::sk_unique);
+        ex = ast::expr_vec(es, mut, ast::sk_rc);
     } else if (p.peek() == token::POUND_LT) {
         p.bump();
         let ty = parse_ty(p);