analyzer: fix leak suppression at end of 'main' [PR101983]

PR analyzer/101983 reports what I thought were false positives
from -Wanalyzer-malloc-leak, but on closer inspection, the
analyzer is correctly reporting heap-allocated buffers that are
no longer reachable.

However, these "leaks" occur at the end of "main".  The analyzer already
has some logic to avoid reporting leaks at the end of main, where the
leak is detected at the end of the EXIT basic block.  However, in this case,
the leak is detected at the clobber in BB 2 here:
  <bb 2> :
  func (&res);
  res ={v} {CLOBBER(eol)};
  _4 = 0;

  <bb 3> :
<L0>:
  return _4;

where we have a chain BB 2 -> BB 3 -> EXIT BB.

This patch generalizes the "are we at the end of 'main'" detection to
handle such cases, silencing -Wanalyzer-malloc-leak on them.

There's a remaining issue where the analyzer unhelpfully describes one
of the leaking values as '<unknown>', rather than 'res.a', but I'm
leaving that for a followup (covered by PR analyzer/99771).

gcc/analyzer/ChangeLog:
	PR analyzer/101983
	* engine.cc (returning_from_function_p): New.
	(impl_region_model_context::on_state_leak): Use it when rejecting
	leaks at the return from "main".

gcc/testsuite/ChangeLog:
	PR analyzer/101983
	* gcc.dg/analyzer/pr101983-main.c: New test.
	* gcc.dg/analyzer/pr101983-not-main.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>