Handle -Wsign-conversion in conversion_warning.

Handle -Wsign-conversion in conversion_warning.

It seemed strange to me to warn about sign conversion in
unsafe_conversion_p, when other warnings are in conversion_warning, and the
latter function is the only place that asks the former function to warn.
This change is also necessary for my -Warith-conversion patch.

	* c-common.c (unsafe_conversion_p): Don't warn, return UNSAFE_SIGN.
	* c-warn.c (conversion_warning): Warn about UNSAFE_SIGN.