[Ada] Remove redundant checks for empty lists

[Pierre-Marie de Rodat]

When iterating over list elements with First/Next there is no need to
detect empty or non-existing list, because First intentionally returns
Empty in those cases.

Cleanup of code related to Unchecked_Unions; behaviour is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch4.adb (Variant_Is_Unconstrained_UU): Remove redundant
	check for empty list.
	* exp_disp.adb (Find_Entry_Index): Simplify by removing
	redundant check and counting from zero; fix type of a local
	variable.
	* sem_ch12.adb (Save_Global_Descendant): Remove an unnecessary
	special-case for empty lists.
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