c++: Fix printing of decltype(nullptr) [PR97517]

c++: Fix printing of decltype(nullptr) [PR97517]

The C++ printer doesn't handle NULLPTR_TYPE, so we issue the ugly
"'nullptr_type' not supported by...".  Since NULLPTR_TYPE is
decltype(nullptr), it seemed reasonable to handle it where we
handle DECLTYPE_TYPE, that is, in the simple-type-specifier handler.

gcc/cp/ChangeLog:

	PR c++/97517
	* cxx-pretty-print.c (cxx_pretty_printer::simple_type_specifier): Handle
	NULLPTR_TYPE.
	(pp_cxx_type_specifier_seq): Likewise.
	(cxx_pretty_printer::type_id): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/97517
	* g++.dg/diagnostic/nullptr.C: New test.