Improve performance of queries like
  SELECT * FROM t1 WHERE field = NAME_CONST('a', 4);
by, in this example, replacing the WHERE clause with field = 4
in the case of ref access.

The rewrite is done during fix_fields and we disambiguate this
case from other cases of NAME_CONST by inspecting where we are
in parsing.  We rely on THD::where to accomplish this.  To
improve performance there, we change the type of THD::where to
be an enumeration, so we can avoid string comparisons during
Item_name_const::fix_fields.  Consequently, this patch also
changes all usages of THD::where to conform likewise.