analyzer: fix reproducer for PR 93375

Reproducing the ICE in PR analyzer/93375 required some kind of
analyzer diagnostic occurring after a call with fewer arguments
than required by the callee.

The testcase used __builtin_memcpy with a NULL argument for this.

On x86_64-pc-linux-gnu this happened to be already optimized into:
  _4 = MEM <unsigned int> [(char * {ref-all})0B];
  MEM <unsigned int> [(char * {ref-all})rl_1] = _4;
by the time of the analyzer pass, leading to the diagnostic in question
being:
  warning: dereference of NULL ‘rl’ [CWE-690] [-Wanalyzer-null-dereference]

On other targets e.g. arm-unknown-linux-gnueabi, the builtin isn't
optimized at the time of the analyzer pass, leading to this diagnostic
instead:
  warning: use of NULL ‘rl’ where non-null expected [CWE-690] [-Wanalyzer-null-argument]
  <built-in>: note: argument 1 of ‘__builtin_memcpy’ must be non-null

This patch fixes the test case by using a custom function marked as
nonnull.  I manually verified that it still reproduces the ICE if the
patch for the PR is reverted.

gcc/testsuite/ChangeLog:
	PR analyzer/93375
	* gcc.dg/analyzer/pr93375.c: Rework test case to avoid per-target
	differences in how __builtin_memcpy has been optimized at the time
	the analyzer runs.

gcc/testsuite/ChangeLog

 2020-02-06  David Malcolm  <dmalcolm@redhat.com>
 
+	PR analyzer/93375
+	* gcc.dg/analyzer/pr93375.c: Rework test case to avoid per-target
+	differences in how __builtin_memcpy has been optimized at the time
+	the analyzer runs.
+
+2020-02-06  David Malcolm  <dmalcolm@redhat.com>
+
 	* gcc.dg/analyzer/torture/intptr_t.c: New test.
 
 2020-02-06  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/gcc.dg/analyzer/pr93375.c

 /* { dg-additional-options "-Wno-implicit-int" } */
 
+extern void foo (void *) __attribute__((nonnull));
+
 void
 en (jm)
 {
@@ -11,5 +13,5 @@ p2 ()
   char *rl = 0;
 
   en ();
-  __builtin_memcpy (rl, 0, sizeof (0)); /* { dg-warning "dereference of NULL" } */
+  foo (rl); /* { dg-warning "use of NULL 'rl' where non-null expected" } */
 }