This implements LWG 2503, which allows ^ and $ to match line terminator
characters, rather than only matching the beginning and end of the
entire input. The multiline option is only valid for ECMAScript, but
for other grammars we ignore it rather than throwing an exception.

This is related to PR libstdc++/102480, which incorrectly said that
ECMAscript should match the beginning of a line when match_prev_avail
is used. I think that's only supposed to happen when multiline is used.

The new regex_constants::multiline and basic_regex::multiline constants
are not defined for strict -std=c++11 and -std=c++14 modes, but
regex_constants::__multiline is always defined, so that the
implementation can use it internally.

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

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (basic_regex::multiline): Define constant
	for C++17.
	* include/bits/regex_constants.h (regex_constants::multiline):
	Define constant for C++17.
	(regex_constants::__multiline): Define duplicate constant for
	internal use in C++11 and C++14.
	* include/bits/regex_executor.h (_Executor::_M_match_multiline()):
	New member function.
	(_Executor::_M_is_line_terminator(_CharT)): New member function.
	(_Executor::_M_at_begin(), _Executor::_M_at_end()): Use new
	member functions to support multiline matches.
	* testsuite/28_regex/algorithms/regex_match/multiline.cc: New test.

Tested x86_64-linux. Committed to trunk.