PR libstdc++/58265 add noexcept to basic_string::assign(basic_string&&)

PR libstdc++/58265 add noexcept to basic_string::assign(basic_string&&)

	PR libstdc++/58265
	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string::assign(basic_string&&)): Add conditional noexcept
	depending on the allocator's is_always_equal property (LWG 2063).
	* testsuite/21_strings/basic_string/modifiers/assign/char/
	move_assign.cc: Check for non-throwing exception specification.
	* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/
	move_assign.cc: Likewise.

From-SVN: r262447