diagnostics: move diagnostic_info to its own header

diagnostics: move diagnostic_info to its own header

No functional change intended.

gcc/ChangeLog:
	* diagnostic.h (struct diagnostic_info): Move to
	diagnostics/diagnostic-info.h as diagnostics::diagnostic_info.
	Add typedef bringing it back into root namespace, for now.
	* diagnostics/diagnostic-info.h: New file, based on the above.
	* langhooks-def.h: Update for diagnostic_info moving into
	namespace diagnostics.
	* langhooks.h: Likewise.

gcc/jit/ChangeLog:
	* jit-playback.cc: Update for diagnostic_info moving into
	namespace diagnostics.
	* jit-playback.h: Likewise.

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