* include/bits/basic_string.h (basic_string::at): Move adjacent to other
	overload.
	(basic_string::pop_back): Define.
	* include/debug/string (__gnu_debug::basic_string::pop_back): Likewise.
	* include/ext/vstring.h (__versa_string::pop_back): Likewise.
	* config/abi/pre/gnu.ver: Add new symbols.
	* testsuite/21_strings/basic_string/modifiers/char/pop_back.cc: New.
	* testsuite/21_strings/basic_string/modifiers/wchar_t/pop_back.cc: New.
	* testsuite/21_strings/basic_string/range_access.cc: Split to ...
	* testsuite/21_strings/basic_string/range_access/char/1.cc: Here and ...
	* testsuite/21_strings/basic_string/range_access/wchar_t/1.cc: Here.
	* testsuite/ext/vstring/modifiers/char/pop_back.cc: New.
	* testsuite/ext/vstring/modifiers/wchar_t/pop_back.cc: New.

From-SVN: r181049