libstdc++: Add __gnu_debug::basic_string<>::compare overloads

libstdc++: Add __gnu_debug::basic_string<>::compare overloads

Rather than adding those implementations we are adding a:
using _Base::compare;

so that any compare method not implemented at __gnu_debug::basic_string
level are injected from the base class.

Also review how __gnu_debug::basic_string is tested. Now require to define
_GLIBCXX_TEST_DEBUG_STRING when running 'make check-debug'.

libstdc++-v3/ChangeLog

	* include/debug/string: Add using _Base::compare.
	(__gnu_debug::basic_string<>::compare(const basic_string<>&)): Remove.
	(__gnu_debug::basic_string<>::compare(size_type, size_type, const basic_string<>&)):
	Remove.
	(__gnu_debug::basic_string<>::compare(size_type, size_type, const basic_string<>&,
	size_type, size_type)): Remove.
	* testsuite/util/testsuite_string.h [_GLIBCXX_TEST_DEBUG_STRING]: Include <debug/string>.
	* testsuite/21_strings/basic_string/operations/compare/char/1.cc: Include testsuite_string.h
	and use __gnu_test::string.
	* testsuite/21_strings/basic_string/operations/compare/char/13650.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/compare/char/2.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/rfind/char/1.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/rfind/char/2.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/rfind/char/3.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/1.cc:
	Include testsuite_string.h
	and use __gnu_test::wstring.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/13650.cc: Likewise.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/2.cc: Likewise.