ada: More Tbuild cleanup

ada: More Tbuild cleanup

Remove "Nmake_Assert => ..." on N_Unchecked_Type_Conversion at
gen_il-gen-gen_nodes.adb:473 (was disabled).

This was left over from commit 82a794419a00ea98b68d69b64363ae6746710de9
"Tbuild cleanup".

In addition, the checks for "Is_Composite_Type" in
Tbuild.Unchecked_Convert_To are narrowed to "not Is_Scalar_Type";
that way, useless duplicate unchecked conversions of access types will
be removed as for composite types.

gcc/ada/ChangeLog:

	* gen_il-gen-gen_nodes.adb (N_Unchecked_Type_Conversion):
	Remove useless Nmake_Assert.
	* tbuild.adb (Unchecked_Convert_To):
	Narrow the bitfield-related conditions.