analyzer: fix "when 'strchr' returns non-NULL" message

analyzer: fix "when 'strchr' returns non-NULL" message

Tweak analyzer handling of strchr, so that we show the
  when 'strchr' returns non-NULL
message for that execution path.

gcc/analyzer/ChangeLog:
	* region-model-impl-calls.cc (region_model::impl_call_strchr):
	Move to on_call_post.  Handle both outcomes using bifurcation,
	rather than just the "not found" case.
	* region-model.cc (region_model::on_call_pre): Move
	BUILT_IN_STRCHR and "strchr" to...
	(region_model::on_call_post): ...here.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/strchr-1.c (test_literal): Detect writing to a
	string literal.  Verify that we emit the "when '__builtin_strchr'
	returns non-NULL" message.

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