analyzer: fix ICE on unary ops folding to casts of constants [PR104089]

gcc/analyzer/ChangeLog:
	PR analyzer/104089
	* region-model-manager.cc
	(region_model_manager::get_or_create_constant_svalue): Assert that
	we have a CONSTANT_CLASS_P.
	(region_model_manager::maybe_fold_unaryop): Only fold a constant
	when fold_unary's result is a constant or a cast of a constant.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/pr104089.c: New test.
	PR analyzer/104089

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