[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.