c++: ICE with operator new[] in constexpr [PR118775]

c++: ICE with operator new[] in constexpr [PR118775]

Here we ICE since r11-7740 because we no longer say that (long)&a
(where a is a global var) is non_constant_p.  So VERIFY_CONSTANT
does not return and we crash on tree_to_uhwi.  We should check
tree_fits_uhwi_p before calling tree_to_uhwi.

	PR c++/118775

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_call_expression): Check tree_fits_uhwi_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-new24.C: New test.
	* g++.dg/cpp2a/constexpr-new25.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>