tree-object-size: Support strndup and strdup

Use string length of input to strdup to determine the usable size of the
resulting object.  Avoid doing the same for strndup since there's a
chance that the input may be too large, resulting in an unnecessary
overhead or worse, the input may not be NULL terminated, resulting in a
crash where there would otherwise have been none.

gcc/ChangeLog:

	* tree-object-size.cc (todo): New variable.
	(object_sizes_execute): Use it.
	(strdup_object_size): New function.
	(call_object_size): Use it.

gcc/testsuite/ChangeLog:

	* gcc.dg/builtin-dynamic-object-size-0.c (test_strdup,
	test_strndup, test_strdup_min, test_strndup_min): New tests.
	(main): Call them.
	* gcc.dg/builtin-dynamic-object-size-1.c: Silence overread
	warnings.
	* gcc.dg/builtin-dynamic-object-size-2.c: Likewise.
	* gcc.dg/builtin-dynamic-object-size-3.c: Likewise.
	* gcc.dg/builtin-dynamic-object-size-4.c: Likewise.
	* gcc.dg/builtin-object-size-1.c: Silence overread warnings.
	Declare free, strdup and strndup.
	(test11): New test.
	(main): Call it.
	* gcc.dg/builtin-object-size-2.c: Silence overread warnings.
	Declare free, strdup and strndup.
	(test9): New test.
	(main): Call it.
	* gcc.dg/builtin-object-size-3.c: Silence overread warnings.
	Declare free, strdup and strndup.
	(test11): New test.
	(main): Call it.
	* gcc.dg/builtin-object-size-4.c: Silence overread warnings.
	Declare free, strdup and strndup.
	(test9): New test.
	(main): Call it.