diagnostics: use unique_ptr for m_format_postprocessor

diagnostics: use unique_ptr for m_format_postprocessor

No functional change intended.

gcc/cp/ChangeLog:
	* error.cc (cxx_format_postprocessor::clone): Update to use
	unique_ptr.
	(cxx_dump_pretty_printer::cxx_dump_pretty_printer): Likewise.
	(cxx_initialize_diagnostics): Likewise.

gcc/ChangeLog:
	* pretty-print.cc (pretty_printer::pretty_printer): Use "nullptr"
	rather than "NULL".  Remove explicit delete of
	m_format_postprocessor.
	* pretty-print.h (format_postprocessor::clone): Use unique_ptr.
	(pretty_printer::set_format_postprocessor): New.
	(pretty_printer::m_format_postprocessor): Use unique_ptr.
	(pp_format_postprocessor): Update for use of unique_ptr, removing
	reference from return type.

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