[Ada] Get rid of more references to Universal_Integer in expanded code

[Ada] Get rid of more references to Universal_Integer in expanded code

2020-06-02  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_aggr.adb (Build_Array_Aggr_Code): Set the type of the PAT
	on the zero used to clear the array.
	* exp_attr.adb (Expand_N_Attribute_Reference)
	<Attribute_Alignment>: In the CW case, directly convert from the
	alignment's type to the target type if the parent is an
	unchecked conversion.
	* sem_res.adb (Set_String_Literal_Subtype): In the dynamic case,
	use the general expression for the upper bound only when needed.
	Set the base type of the index as the type of the low bound.
	(Simplify_Type_Conversion): Do an intermediate conversion to the
	root type of the target type if the operand is an integer
	literal.
	* tbuild.adb (Convert_To): Get rid of an intermediate conversion
	to Universal_Integer if the inner expression has integer tyoe.
	* libgnat/a-sequio.adb (Byte_Swap): Make use of an equivalent
	static expression in the case statement.