* c-typeck.c (convert_for_assignment):
Bring back conversion to union without a cast,
undoing the Jan 16 change, but with the following differences:

* The union must have the transparent_union attribute.
* The conversion must be for a function argument.
* Warn consistently about such conversions if pedantic.
* Do not warn about an assignment incompatibility for one union member
  if another union member is compatible with no warning.

From-SVN: r11290