diagnostics: xml: add add_text_from_pp

diagnostics: xml: add add_text_from_pp

Various places use
  xp.add_text (pp_formatted_text (&pp))
Add a helper function for this.
No functional change intended.

gcc/ChangeLog:
	* diagnostic-path-output.cc: Use xml::printer::add_text_from_pp.
	* diagnostic-show-locus.cc: Likewise.
	* xml-printer.h (xml::printer::add_text_from_pp): New decl.
	* xml.cc (xml::node_with_children::add_text_from_pp): New.
	(xml::printer::add_text_from_pp): New.
	* xml.h (xml::node_with_children::add_text_from_pp): New decl.

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