[Ada] INOX: prototype "when" constructs

[Ada] INOX: prototype "when" constructs

[Pierre-Marie de Rodat]

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This patch implements experimental features under the -gnatX flag for
"return ... when", "raise ... when", and "goto ... when" constructs.

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

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Document new
	feature under pragma Extensions_Allowed.
	* gnat_rm.texi: Regenerate.
	* errout.adb, errout.ads (Error_Msg_GNAT_Extension): Created to
	issue errors when parsing extension only constructs.
	* exp_ch11.adb, exp_ch11.ads (Expand_N_Raise_When_Statement):
	Created to expand raise ... when constucts.
	* exp_ch5.adb, exp_ch5.ads (Expand_N_Goto_When_Statement):
	Created to expand goto ... when constructs.
	* exp_ch6.adb, exp_ch6.ads (Expand_N_Return_When_Statement):
	Created to expand return ... when constructs.
	* expander.adb (Expand): Add case entries for "when" constructs.
	* gen_il-gen-gen_nodes.adb, gen_il-types.ads: Add entries for
	"when" constructs.
	* par-ch11.adb (P_Raise_Statement): Add processing for raise ...
	when.
	* par-ch5.adb (Missing_Semicolon_On_Exit): Renamed to
	Missing_Semicolon_On_When and moved to par-util.adb.
	* par-ch6.adb (Get_Return_Kind): Renamed from Is_Simple and
	processing added for "return ... when" return kind.
	(Is_Simple): Renamed to Get_Return_Kind.
	(P_Return_Statement): Add case for return ... when variant of
	return statement.
	* par-util.adb, par.adb (Missing_Semicolon_On_When): Added to
	centeralize parsing of "when" keywords in the context of "when"
	constructs.
	* sem.adb (Analyze): Add case for "when" constructs.
	* sem_ch11.adb, sem_ch11.ads (Analyze_Raise_When_Statement):
	Created to analyze raise ... when constructs.
	* sem_ch5.adb, sem_ch5.ads (Analyzed_Goto_When_Statement):
	Created to analyze goto ... when constructs.
	* sem_ch6.adb, sem_ch6.ads (Analyze_Return_When_Statement):
	Created to analyze return ... when constructs.
	* sprint.adb (Sprint_Node_Actual): Add entries for new "when"
	nodes.

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