ada: update Ada_Version_Type in fe.h to match opt.ads

ada: update Ada_Version_Type in fe.h to match opt.ads

Remove Ada_With_Extensions, which is not used on the C side.
Do not add Ada_With_Core_Extensions and Ada_With_All_Extensions,
which are also not used on the C side, and on the Ada side
are always used via functions All_Extensions_Allowed and
Core_Extensions_Allowed. Explain this in comments.

Move the functions closer to the type declaration,
so the usage style is clearer.

Cleanup only -- no change in compiler behavior.

gcc/ada/

	* fe.h: Remove Ada_With_Extensions and add commentary.
	* opt.ads: Rearrange code and add commentary.