c++: Allow constexpr reads from volatile std::nullptr_t objects [PR118661]

c++: Allow constexpr reads from volatile std::nullptr_t objects [PR118661]

As mentioned in the PR, https://eel.is/c++draft/conv.lval#note-1
says that even volatile reads from std::nullptr_t typed objects actually
don't read anything and https://eel.is/c++draft/expr.const#10.9
says that even those are ok in constant expressions.

So, the following patch adjusts the r9-4793 changes to have an exception
for NULLPTR_TYPE.
As [conv.lval]/3 also talks about accessing to inactive member, I've added
testcase to cover that as well.

2025-02-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/118661
	* constexpr.cc (potential_constant_expression_1): Don't diagnose
	lvalue-to-rvalue conversion of volatile lvalue if it has NULLPTR_TYPE.
	* decl2.cc (decl_maybe_constant_var_p): Return true for constexpr
	decls with NULLPTR_TYPE even if they are volatile.

	* g++.dg/cpp0x/constexpr-volatile4.C: New test.
	* g++.dg/cpp0x/constexpr-union9.C: New test.