[Ada] Check all pairs of actuals for overlapping

[Pierre-Marie de Rodat]

When detecting overlapping actuals GNAT was giving up on the first pair
of parameters that refer to the same object (even when there was no
overlapping because both parameters had mode IN). Now we examine all
pairs.

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

gcc/ada/

	* sem_warn.adb (Warn_On_Overlapping_Actuals): Fix style;
	refactor repeated calls to Nkind; remove early RETURN.
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