libstdc++/regex: Avoid -Wunused-parameter warnings in _Executor

libstdc++/regex: Avoid -Wunused-parameter warnings in _Executor

Now that _Executor is non-recursive most subroutines no longer use their
_Match_mode parameter (previously it was just passed to recursive _M_dfs
calls).  For now, just make them unnamed to avoid warnings.

libstdc++-v3/ChangeLog:

	* include/bits/regex_executor.tcc (_Executor::_M_rep_once_more):
	Make unnused _Match_mode parameter unnamed.
	(_Executor::_M_handle_repeat): Likewise.
	(_Executor::_M_handle_subexpr_begin): Likewise.
	(_Executor::_M_handle_subexpr_end): Likewise.
	(_Executor::_M_handle_line_begin_assertion): Likewise.
	(_Executor::_M_handle_line_end_assertion): Likewise.
	(_Executor::_M_handle_match): Likewise.
	(_Executor::_M_handle_backref): Likewise.
	(_Executor::_M_handle_alternative): Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>