diagnostics: minor cleanups

diagnostics: minor cleanups

No functional change intended.

gcc/ChangeLog:
	* diagnostic.h (diagnostics::get_cwe_url): Move decl to
	diagnostics/metadata.h.
	(diagnostics::maybe_line_and_column): Move into
	diagnostics::text_sink.
	* diagnostics/context.cc: Update for maybe_line_and_column
	becoming a static member of text_sink.
	* diagnostics/metadata.h (diagnostics::get_cwe_url): Move decl
	here from diagnostic.h.
	* diagnostics/text-sink.cc (maybe_line_and_column): Convert to...
	(text_sink::maybe_line_and_column): ...this.
	* diagnostics/text-sink.h (text_sink::maybe_line_and_column): Move
	here from diagnostic.h.

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