c++: "'decltype_type' not supported" in diagnostic [PR85278]

c++: "'decltype_type' not supported" in diagnostic [PR85278]

This fixes a garbled concepts diagnostic by moving the handling of DECLTYPE_TYPE
from pp_cxx_type_specifier_seq to cxx_pretty_printer::simple_type_specifier, a
move which also seems to be more consistent with the language grammar.

This patch also fixes pretty printing of rvalue reference types via
cxx_pretty_printer::type_id, which eventually calls pp_c_pointer which currently
doesn't distinguish between lvalue and rvalue references.

gcc/c-family/ChangeLog:

	PR c++/85278
	* c-pretty-print.c (pp_c_pointer) <case REFERENCE_TYPE>: Print a double
	ampersand if it's an rvalue reference type.

gcc/cp/ChangeLog:

	PR c++/85278
	* cxx-pretty-print.c (cxx_pretty_printer:simple_type_specifier)
	<case DECLTYPE_TYPE>: Handle DECLTYPE_TYPE here instead of ...
	(pp_cxx_type_specifier_seq) <case DECLTYPE_TYPE>: ... here.
	(cxx_pretty_printer::direct_abstract_declarator) <case DECLTYPE_TYPE>:
	New no-op case.

gcc/testsuite/ChangeLog:

	PR c++/85278
	* g++.dg/concepts/diagnostic9.C: New test.