scans.ads: New token At_Sign.

scans.ads: New token At_Sign.

2017-01-23  Ed Schonberg  <schonberg@adacore.com>

	* scans.ads: New token At_Sign. Remove '@' from list of illegal
	characters for future version of the language. '@' is legal name.
	* scng.ads, scng.adb (Scan):  Handle '@' appropriately.
	* scn.adb (Scan_Reserved_Identifier): An occurrence of '@'
	denotes a Target_Name.
	* par-ch4.adb (P_Name, P_Primary): Handle Target_Name.
	* sinfo.ads, sinfo.adb (N_Target_Name): New non-terminal node.
	(Has_Target_Names): New flag on N_Assignment_Statement, to
	indicate that RHS has occurrences of N_Target_Name.
	* sem.adb: Call Analyze_Target_Name.
	* sem_ch5.ads, sem_ch5.adb (Analyze_Target_Name): New subpogram.
	(urrent_LHS): Global variable that denotes LHS of assignment,
	used in the analysis of Target_Name nodes.
	* sem_res.adb (Resolve_Target_Name): New procedure.
	* exp_ch5.adb (Expand_Assign_With_Target_Names): (AI12-0125):
	N is an assignment statement whose RHS contains occurences of @
	that designate the value of the LHS of the assignment. If the
	LHS is side-effect free the target names can be replaced with
	a copy of the LHS; otherwise the semantics of the assignment
	is described in terms of a procedure with an in-out parameter,
	and expanded as such.
	(Expand_N_Assignment_Statement): Call
	Expand_Assign_With_Target_Names when needed.
	* exp_util.adb (Insert_Actions): Take into account N_Target_Name.
	* sprint.adb: Handle N_Target_Name.

From-SVN: r244783