[Ada] Fix violation of No_Implicit_Loops restriction for enumeration type

[Pierre-Marie de Rodat]

The perfect hash function generated by the compiler to speed up the Value
attribute of an enumeration type contains an implicit loop and, therefore,
violates the No_Implicit_Loops restriction when it is active.

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

gcc/ada/

	* exp_imgv.adb: Add with and use clause for Restrict and Rident.
	(Build_Enumeration_Image_Tables): Do not generate the hash function
	if the No_Implicit_Loops restriction is active.
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