Add new std::basic_string constructor (LWG 2583)

Add new std::basic_string constructor (LWG 2583)

	* config/abi/pre/gnu.ver (GLIBCXX_3.4, GLIBCXX_3.4.21): Use more
	precise patterns for basic_string constructors.
	(GLIBCXX_3.4.23): Export new constructors.
	* doc/xml/manual/intro.xml: Document LWG 2583 status.
	* doc/html/*: Regenerate.
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string(const basic_string&, size_type, const Alloc&)): Add
	new constructor for LWG 2583.
	(basic_string(const basic_string&, size_type, size_type)): Remove
	default argument.
	[!_GLIBCXX_USE_CXX11_ABI]: Likewise.
	* include/bits/basic_string.tcc [!_GLIBCXX_USE_CXX11_ABI]: Define it.
	* testsuite/21_strings/basic_string/cons/char/8.cc: New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/8.cc: New test.

From-SVN: r239773