PR middle-end/93646 - confusing -Wstringop-truncation on strncat where -Wstringop-overflow is expected

gcc/ChangeLog:

	PR middle-end/93646
	* tree-ssa-strlen.c (handle_builtin_stxncpy): Rename...
	(handle_builtin_stxncpy_strncat): ...to this.  Change first argument.
	Issue only -Wstringop-overflow strncat, never -Wstringop-truncation.
	(strlen_check_and_optimize_call): Adjust callee name.

gcc/testsuite/ChangeLog:

	PR middle-end/93646
	* gcc.dg/Wstringop-overflow-31.c: New test.