[Ada] Replace Opt.Extensions_Allowed by Ada_Version

[Pierre-Marie de Rodat]

Code cleanup: add a new value in Ada_Version_Type to represent the
latest version of Ada with GNAT extensions instead of using a separate
flag.

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

gcc/ada/

	* fe.h, opt.adb, opt.ads, par-prag.adb, sem_prag.adb,
	switch-c.adb (Extensions_Allowed): Replace by a function.
	(Ada_Version_Type): Add new value Ada_With_Extensions, to
	replace setting of Extensions_Allowed.  Update setting of
	Extensions_Allowed.
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