libstdc++: std::string move assignment should not use POCCA trait [PR116641]

The changes to implement LWG 2579 (r10-327-gdb33efde17932f) made
std::string::assign use the propagate_on_container_copy_assignment
(POCCA) trait, for consistency with operator=(const basic_string&).
However, this also unintentionally affected operator=(basic_string&&)
which calls assign(str) to make a deep copy when performing a move is
not possible. The fix is for the move assignment operator to call
_M_assign(str) instead of assign(str), as this just does the deep copy
and doesn't check the POCCA trait first.

The bug only affects the unlikely/useless combination of POCCA==true and
POCMA==false, but we should fix it for correctness anyway. it should
also make move assignment slightly cheaper to compile and execute,
because we skip the extra code in assign(const basic_string&).

libstdc++-v3/ChangeLog:

	PR libstdc++/116641
	* include/bits/basic_string.h (operator=(basic_string&&)): Call
	_M_assign instead of assign.
	* testsuite/21_strings/basic_string/allocator/116641.cc: New
	test.