[Ada] Further adjustment and optimization of System.Value_N

[Ada] Further adjustment and optimization of System.Value_N

gcc/ada/

	* rtsfind.ads (RE_Id): Change RE_Valid_Enumeration_Value_NN into
	RE_Valid_Value_Enumeration_NN.
	(RE_Unit_Table): Adjust to above renaming.
	* exp_imgv.adb (Expand_Valid_Value_Attribute): Likewise.
	* libgnat/s-valuen.ads (Invalid): Remove.
	(Value_Enumeration_Pos): Move to...
	* libgnat/s-valuen.adb (Value_Enumeration_Pos): ...here.
	Return -1 instead of Invalid.
	(Value_Enumeration): Compare against 0 instead of Invalid.
	(Valid_Enumeration_Value): Likewise.  Rename to...
	(Valid_Value_Enumeration): ...this.
	* libgnat/s-vaenu8.ads (Valid_Enumeration_Value_8): Rename into...
	(Valid_Value_Enumeration_8): ...this.
	* libgnat/s-vaen16.ads (Valid_Enumeration_Value_16): Rename into...
	(Valid_Value_Enumeration_16): ...this.
	* libgnat/s-vaen32.ads (Valid_Enumeration_Value_32): Rename into...
	(Valid_Value_Enumeration_32): ...this.