Introduce pretty_printer::clone vfunc

Introduce pretty_printer::clone vfunc

This patch provides a way to clone a pretty_printer.

This is needed so that we can capture text in a label_text and make
layout decisions based on it, using the policy of global_dc's printer,
whilst within a call to diagnostic_show_locus.  We can't print with
the pretty_printer itself within a call to diagnostic_show_locus since
it has partly-buffered content.

gcc/c-family/ChangeLog:
	* c-pretty-print.c (c_pretty_printer::clone): New vfunc
	implementation.
	* c-pretty-print.h (c_pretty_printer::clone): New vfunc decl.

gcc/cp/ChangeLog:
	* cxx-pretty-print.c (cxx_pretty_printer::clone): New vfunc
	implementation.
	* cxx-pretty-print.h (cxx_pretty_printer::clone): New vfunc decl.
	* error.c (cxx_format_postprocessor::clone): New vfunc.

gcc/ChangeLog:
	* pretty-print.c (pretty_printer::pretty_printer): New copy-ctor.
	(pretty_printer::clone): New vfunc implementation.
	* pretty-print.h (format_postprocessor::clone): New pure vfunc
	decl.
	(pretty_printer::pretty_printer): New copy-ctor decl.
	(pretty_printer::clone): New vfunc decl.

From-SVN: r279244