analyzer: avoid using <string.h> in malloc-1.c

analyzer: avoid using <string.h> in malloc-1.c

This test assumes that memset and strlen have been marked with
__attribute__((nonnull)), which isn't necessarily the case for an
arbitrary <string.h>.  This likely explains these failures:
  FAIL: gcc.dg/analyzer/malloc-1.c  (test for warnings, line 417)
  FAIL: gcc.dg/analyzer/malloc-1.c  (test for warnings, line 418)
  FAIL: gcc.dg/analyzer/malloc-1.c  (test for warnings, line 425)
  FAIL: gcc.dg/analyzer/malloc-1.c  (test for warnings, line 429)
seen in https://gcc.gnu.org/ml/gcc-testresults/2020-01/msg01608.html
on x86_64-apple-darwin18.

Fix it by using the __builtin_ forms.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/malloc-1.c: Remove include of <string.h>.
	Use __builtin_ forms of memset and strlen throughout.