ada: Allow enabling a restricted set of language extensions.

ada: Allow enabling a restricted set of language extensions.

The -gnatX switch (and the related Extensions_Allowed pragma) is currently a
two-valued all-or-nothing option. Add support for enabling a curated subset
of language extensions without enabling others via the -gnatX switch
and for enabling all language extensions via the new -gnatX0 switch.
Similarly, the existing "ON" argument for the Extensions_Allowed pragma
now only enables the curated subset; the new argument "ALL" enables all
language extensions. The subset of language extensions currently includes
prefixed-view notation with an untagged prefix, fixed-low-bound array
subtypes, and casing on composite values.

gcc/ada/

	* opt.ads: Replace Ada_Version_Type enumeration literal
	Ada_With_Extensions with two literals, Ada_With_Core_Extensions
	and Ada_With_All_Extensions. Update uses of the deleted literal.
	Replace Extensions_Allowed function with two functions:
	All_Extensions_Allowed and Core_Extensions_Allowed.
	* errout.ads, errout.adb: Add Boolean parameter to
	Error_Msg_GNAT_Extension to indicate whether the construct in
	question belongs to the curated subset.
	* exp_ch5.adb, par-ch4.adb, sem_case.adb, sem_ch3.adb:
	* sem_ch4.adb, sem_ch5.adb, sem_ch8.adb: Replace calls to
	Extensions_Allowed with calls to Core_Extensions_Allowed for
	constructs that are in the curated subset.
	* sem_attr.adb, sem_ch13.adb, sem_eval.adb, sem_util.adb: Replace
	calls to Extensions_Allowed with calls to All_Extensions_Allowed
	for constructs that are not in the curated subset.
	* par-ch3.adb: Override default for new parameter in calls to
	Error_Msg_GNAT_Extension for constructs in the curated subset.
	* par-prag.adb: Add Boolean parameter to Check_Arg_Is_On_Or_Off to
	also allow ALL. Set Opt.Ada_Version appropriately for ALL or ON
	arguments.
	* sem_prag.adb: Allowed ALL argument for an Extensions_Allowed
	pragma. Set Opt.Ada_Version appropriately for ALL or ON arguments.
	* switch-c.adb: The -gnatX switch now enables only the curated
	subset of language extensions (formerly it enabled all of them);
	the new -gnatX0 switch enables all of them.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document new "-gnatX0" switch and update documentation for
	"-gnatX" switch.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Document new ALL
	argument for pragma Extensions_Allowed and update documentation
	for the ON argument. Delete mention of Ada 2022 Reduce attribute
	as an extension.
	* gnat_rm.texi, gnat_ugn.texi: Regenerate.