libstdc++: Add string contains member functions for C++2b

This implements WG21 P1679R3, adding contains member functions to
basic_string_view and basic_string.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::contains): New
	member functions.
	* include/std/string_view (basic_string_view::contains):
	Likewise.
	* include/std/version (__cpp_lib_string_contains): Define.
	* testsuite/21_strings/basic_string/operations/starts_with/char/1.cc:
	Remove trailing whitespace.
	* testsuite/21_strings/basic_string/operations/starts_with/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/contains/char/1.cc: New test.
	* testsuite/21_strings/basic_string/operations/contains/wchar_t/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/contains/char/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/contains/char/2.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/contains/wchar_t/1.cc: New test.