diagnostics: introduce context-options.h

diagnostics: introduce context-options.h

No functional change intended.

gcc/ChangeLog:
	* diagnostic.h (enum diagnostics_column_unit): Move to
	diagnostics/context-options.h.
	(enum diagnostics_escape_format): Likewise.
	(enum diagnostics_output_format): Likewise.
	(enum diagnostic_path_format): Likewise.
	(enum diagnostics_extra_output_kind): Likewise.
	(enum diagnostic_text_art_charset): Likewise.
	* diagnostics/context-options.h: New file, based on the above
	material.
	* diagnostics/context.h: Include it.

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