PR libstdc++/58265 implement LWG 2063 for COW strings

PR libstdc++/58265 implement LWG 2063 for COW strings

For COW strings the default constructor does not allocate when
_GLIBCXX_FULLY_DYNAMIC_STRING == 0, so can be noexcept. The move
constructor and swap do not allocate when the allocators are equal, so
add conditional noexcept using allocator_traits::is_always_equal.

	PR libstdc++/58265
	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	[_GLIBCXX_FULLY_DYNAMIC_STRING==0] (basic_string::basic_string()):
	Add GLIBCXX_NOEXCEPT.
	(basic_string::operator=(basic_string&&)): Add _GLIBCXX_NOEXCEPT_IF
	to depend on the allocator's is_always_equal property (LWG 2063).
	(basic_string::swap(basic_string&)): Likewise.
	* include/bits/basic_string.tcc [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string::swap(basic_string&)): Likewise.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc:
	Check is_nothrow_move_assignable.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move_assign.cc:
	Check is_nothrow_move_assignable.
	* testsuite/21_strings/basic_string/cons/char/
	noexcept_move_construct.cc: Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/
	noexcept_move_construct.cc: Likewise.

From-SVN: r262443