[Ada] CUDA: Use internal types instead of public ones

[Ada] CUDA: Use internal types instead of public ones

gcc/ada/

	* exp_prag.adb (Get_Launch_Kernel_Arg_Type): Renamed to
	Get_Nth_Arg_Type and made more generic.
	(Build_Dim3_Declaration): Now builds a CUDA.Internal.Dim3
	instead of a CUDA.Vector_Types.Dim3.
	(Build_Shared_Memory_Declaration): Now infers needed type from
	Launch_Kernel instead of using a hard-coded type.
	(Expand_Pragma_CUDA_Execute): Build additional temporaries to
	store Grids and Blocks.
	* rtsfind.ads: Move Launch_Kernel from public to internal
	package.