[Ada] Fix couple of bugs in the implementation of Round attribute

[Ada] Fix couple of bugs in the implementation of Round attribute

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference) <Attribute_Round>:
	Adjust commentary and set the Rounded_Result flag on the type
	conversion node when the node is needed.
	* exp_ch4.adb (Expand_N_Type_Conversion): Minor tweak.
	(Fixup_Universal_Fixed_Operation): Look through the type conversion
	only when it is to Universal_Real.
	* exp_fixd.adb: Remove with and use clauses for Snames.
	(Build_Divide): Remove redundant test.
	(Expand_Convert_Float_To_Fixed): Use Rounded_Result flag on the
	node to set the truncation parameter.