ada: Small adjustments to new procedure Expand_Unchecked_Union_Equality

The procedure is not stable under repeated invocation.  Now it may be called
twice on the same node, for example during the expansion of the renaming of
the predefined equality operator after the unchecked union type is frozen.

gcc/ada/

	* exp_ch4.ads (Expand_Unchecked_Union_Equality): Only take a
	single parameter.
	* exp_ch4.adb (Expand_Unchecked_Union_Equality): Add guard against
	repeated invocation on the same node.
	* exp_ch6.adb (Expand_Call): Only pass a single actual parameter
	in the call to Expand_Unchecked_Union_Equality.