libstdc++: Optimize basic_string move assignment

libstdc++: Optimize basic_string move assignment

Since resolution of Issue 2593 [1] we can consider that equal allocators
before the propagate-on-move-assignment operations will still be equal
afterward.

So we can extend the optimization of transfering the storage of the move-to
instance to the move-from one that is currently limited to always equal
allocators.

[1] https://cplusplus.github.io/LWG/issue2593

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator=(basic_string&&)): Transfer move-to
	storage to the move-from instance when allocators are equal.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc (test04):
	New test case.