libstdc++: Add [[nodiscard]] attribute to <string> and <string_view>

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string): Add nodiscard
	attribute to all relevant functions.
	* include/std/string_view (basic_string_view): Likewise.
	* testsuite/21_strings/basic_string/capacity/1.cc: Cast unused
	results to void.
	* testsuite/21_strings/basic_string/capacity/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/char/self_move.cc:
	Likewise.
	* testsuite/21_strings/basic_string/element_access/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/element_access/char/21674.cc:
	Likewise.
	* testsuite/21_strings/basic_string/element_access/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/element_access/wchar_t/21674.cc:
	Likewise.
	* testsuite/21_strings/basic_string/lwg2758.cc: Likewise.
	* testsuite/21_strings/basic_string/lwg2946.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/contains/nonnull.cc:
	Add -Wno-unused-result to options.
	* testsuite/21_strings/basic_string/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operators/char/1.cc: Cast
	unused results to void.
	* testsuite/21_strings/basic_string/operators/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/capacity/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/char/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/element_access/wchar_t/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/contains/nonnull.cc:
	Likewise.
	Add -Wno-unused-result to options.
	* testsuite/21_strings/basic_string_view/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/sputbackc/char/1.cc: Cast
	unused results to void.
	* testsuite/27_io/basic_stringbuf/sputbackc/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/sungetc/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/sungetc/wchar_t/1.cc:
	Likewise.