analyzer: fix reproducer for PR 93375

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.