[Ada] Adjust expansion of perfect hash function for Value

[Pierre-Marie de Rodat]

This moves the declaration of the 4 tables to inside the function itself
for reasons explained in the code.

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

gcc/ada/

	* exp_imgv.ads (Build_Enumeration_Image_Tables): Adjust comment.
	* exp_imgv.adb (Build_Enumeration_Image_Tables): Add the
	declaration nodes of the 4 tables to the declaration list of the
	function body.
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