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

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

	PR libstdc++/70483
	* include/experimental/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/experimental/string_view (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, operator==, operator!=)
	(operator<, operator>, operator<=, operator>=): Likewise.
	* testsuite/experimental/string_view/operations/compare/char/70483.cc:
	New.

From-SVN: r252017