Implement N4258 noexcept for std::basic_string.

Implement N4258 noexcept for std::basic_string.

	PR libstdc++/58265
	* doc/xml/manual/intro.xml: Document LWG 2063 and 2064 resolutions.
	* doc/html/manual/bugs.html: Regenerate.
	* include/bits/basic_string.h (basic_string): Implement N4258. Add
	correct exception-specifications and propagate allocators correctly.
	* include/bits/basic_string.tcc (basic_string::swap): Propagate
	allocators correctly.
	* include/debug/string (__gnu_debug::basic_string): Add correct
	exceptions-specifications and allcoator-extended constructors.
	* testsuite/21_strings/basic_string/allocator/char/copy.cc: New.
	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
	New.
	* testsuite/21_strings/basic_string/allocator/char/minimal.cc: New.
	* testsuite/21_strings/basic_string/allocator/char/move.cc: New.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc:
	New.
	* testsuite/21_strings/basic_string/allocator/char/noexcept.cc: New.
	* testsuite/21_strings/basic_string/allocator/char/swap.cc: New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy.cc: New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
	New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/minimal.cc: New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move.cc: New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move_assign.cc:
	New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/noexcept.cc: New.
	* testsuite/21_strings/basic_string/allocator/wchar_t/swap.cc: New.
	* testsuite/util/testsuite_allocator.h (tracker_allocator): Define
	defaulted assignment operators.

From-SVN: r227681