[Ada] Legality checks involving aspect Import

[Arnaud Charlet <charlet at adacore dot com>]

This patch adds some missing legality checks on uses of aspect Import. These
checks were performed properly in the presence of pragmas, but were incomplete
when using aspects.

Compiling q.ads must yield:

q.ads:2:28: imported entities cannot have explicit initialization (RM 8.1 (24))
q.ads:6:24: no initialization allowed for declaration of "J" at line 5
q.ads:6:24: imported entities cannot be initialized (RM B.1(24))
q.ads:9:24: no initialization allowed for declaration of "K" at line 8
q.ads:9:24: imported entities cannot be initialized (RM B.1(24))

---
package Q is
   R : constant Integer := 42 with Import, Volatile;
   Q1 : constant := R*R + R*R;

   J : constant Integer := -999 with Volatile;
   pragma Import (Ada, J);

   K : Integer := 1123;
   pragma Import (Ada, K);
end;

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

2014-08-01  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch13.adb (Analyze_Aspect_Specifications, case Aspect_Import):
	Set Is_Imported flag at once, to simplify subsequent legality
	checks. Reject the aspect on an object whose declaration has an
	explicit initial value.
	* sem_prag.adb (Process_Import_Or_Interface): Use original node
	to check legality of an initial value for an imported entity.
	Set Is_Imported flag in case of error to prevent cascaded errors.
	Do not set the Is_Imported flag if the pragma comes from an
	aspect, because it is already done when analyzing the aspect.

Attachment: difs
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