Add json-diagnostic.{cc,h}

Add json-diagnostic.{cc,h}

This patch adds support for emitting diagnostics about JSON input files
to global_dc, showing both the file/line/columns and the JSON Pointer
for the problematic json::value.  Test coverage is added by the followup
on aarch64.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add json-diagnostic.o.
	* diagnostics/client-data-hooks.h
	(class client_data_hooks_decorator): New.
	* diagnostics/context.cc (context::set_client_data_hooks): Return
	the old hooks.
	* diagnostics/context.h (context::set_client_data_hooks): Update
	decl likewise.
	* json-diagnostic.cc: New file.
	* json-diagnostic.h: New file.

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