diagnostics: move buffering code to its own .cc file

diagnostics: move buffering code to its own .cc file

No functional change intended.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add diagnostics/buffering.o.
	* diagnostic.cc (diagnostics::context::set_diagnostic_buffer): Move to
	diagnostics/buffering.cc.
	(diagnostics::context::clear_diagnostic_buffer): Likewise.
	(diagnostics::context::flush_diagnostic_buffer): Likewise.
	(diagnostics::buffer::buffer): Likewise.
	(diagnostics::buffer::~buffer): Likewise.
	(diagnostics::buffer::dump): Likewise.
	(diagnostics::buffer::empty_p): Likewise.
	(diagnostics::buffer::move_to): Likewise.
	(diagnostics::buffer::ensure_per_sink_buffers): Likewise.
	* diagnostics/buffering.cc: New file, from material in diagnostic.cc.
	* diagnostics/buffering.h: Fix comment.
	* diagnostics/text-sink.h: Likewise.

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