PR libstdc++/70483 make std::string_view fully constexpr

PR libstdc++/70483 make std::string_view fully constexpr

	PR libstdc++/70483
	* include/bits/string_view.tcc (basic_string_view::find)
	(basic_string_view::rfind, basic_string_view::find_first_of)
	(basic_string_view::find_last_of, basic_string_view::find_first_not_of)
	(basic_string_view::find_last_not_of): Add constexpr specifier.
	* include/std/string_view (basic_string_view::operator=)
	(basic_string_view::rbegin, basic_string_view::rend)
	(basic_string_view::crbegin, basic_string_view::crend)
	(basic_string_view::remove_prefix, basic_string_view::remove_suffix)
	(basic_string_view::swap, basic_string_view::compare)
	(basic_string_view::find, basic_string_view::rfind)
	(basic_string_view::find_first_of, basic_string_view::find_last_of)
	(basic_string_view::find_first_not_of)
	(basic_string_view::find_last_not_of, basic_string_view::_M_check)
	(basic_string_view::_M_limit, operator==, operator!=, operator<)
	(operator>, operator<=, operator>=): Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	char/1.cc: Repeat tests in constexpr context.
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	char/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/char/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/char/3.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/wchar_t/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/find/wchar_t/3.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/char/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/wchar_t/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/range_access/char/1.cc: Test
	cbegin, cend, rbegin, rend, crbegin and crend.
	* testsuite/21_strings/basic_string_view/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/compare/char/1.cc:
	Remove trailing whitespace.
	* testsuite/21_strings/basic_string_view/operations/compare/wchar_t/
	1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/swap/char/1.cc:
	New.
	* testsuite/21_strings/basic_string_view/modifiers/swap/wchar_t/1.cc:
	New.
	* testsuite/21_strings/basic_string_view/operations/compare/char/2.cc:
	New.
	* testsuite/21_strings/basic_string_view/operations/compare/wchar_t/
	2.cc: New.

From-SVN: r251988