[Ada] Avoid passing Enum_Lit'Size to the back end

[Pierre-Marie de Rodat]

Constant fold Enum_Lit'Size so the back end won't see it.

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

gcc/ada/

	* sem_attr.adb (Eval_Attribute): For Enum_Lit'Size, use
	Enum_Type'Object_Size.
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