Revert previous change: complexN only converts to complexN.

This was discussed with Ken and Russ.  6g has been changed,
and both compilers now work this way.  This avoids a double
type conversion--first to the float type, then to the complex
type.

R=r
CC=golang-dev, ken2, rsc
https://golang.org/cl/419042

doc/go_spec.html

@@ -3262,7 +3262,7 @@ When converting a floating-point number to an integer, the fraction is discarded
 </li>
 <li>
 A value of complex type may be converted to a different complex type,
-but there is no conversion from complex to any other type.
+but there is no conversion between complex and any other type.
 <li>
 When converting a number to a floating-point or complex type,
 the result value is rounded