diagnostics: avoid using "sink" for things that aren't a diagnostics::sink

diagnostics: avoid using "sink" for things that aren't a diagnostics::sink

No functional change intended.

gcc/ChangeLog:
	* diagnostic.h: Use TextOrHtml rather than Sink for template
	params.
	* diagnostics/context.h: Likewise.
	* diagnostics/selftest-context.cc: Use "html_or_text" rather than
	"sink".
	* diagnostics/source-printing.cc: Use TextOrHtml rather than Sink
	for template params.  Use "text_out" and "html_out" rather than
	"sink".

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