[Ada] Ada 2020: Allow declarative items mixed with statements

[Ada] Ada 2020: Allow declarative items mixed with statements

[Pierre-Marie de Rodat]

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This patch implements a syntactic language extension that allows
declarative items to appear in a sequence of statements.  For example:

    for X in S'Range loop
        Item : Character renames S (X);
        Item := Transform (Item);
    end loop;

Previously, declare/begin/end was required, which is just noise.

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

gcc/ada/

	* par.adb (P_Declarative_Items): New function to parse a
	sequence of declarative items.
	(P_Sequence_Of_Statements): Add Handled flag, to indicate
	whether to wrap the result in a block statement.
	* par-ch3.adb (P_Declarative_Item): Rename P_Declarative_Items
	to be P_Declarative_Item, because it really only parses a single
	declarative item, and to avoid conflict with the new
	P_Declarative_Items. Add In_Statements.  We keep the old
	error-recovery mechanisms in place when In_Statements is False.
	When True, we don't want to complain about statements, because
	we are parsing a sequence of statements.
	(P_Identifier_Declarations): If In_Statements, and we see what
	looks like a statement, we no longer give an error. We return to
	P_Sequence_Of_Statements with Done = True, so it can parse the
	statement.
	* par-ch5.adb (P_Sequence_Of_Statements): Call
	P_Declarative_Items to parse declarative items that appear in
	the statement list.  Remove error handling code that complained
	about such items.  Check some errors conservatively.  Wrap the
	result in a block statement when necessary.
	* par-ch11.adb (P_Handled_Sequence_Of_Statements): Pass
	Handled => True to P_Sequence_Of_Statements.
	* types.ads (No, Present): New functions for querying
	Source_Ptrs (equal, not equal No_Location).

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