[Ada] Fix internal error on constant with nested variant part

[Arnaud Charlet <charlet at adacore dot com>]

This fixes an internal error in the gimplifier when it is trying to generate
code for a CONSTRUCTOR of a record type with a component whose type contains
a variant part.

The handling of this kind of CONSTRUCTORs is delicate and the front-end has a
special circuitry to pre-decompose them (or rather aggregates, as they are
called in Ada) into assignments.  This circuitry was recently adjusted to be
less conservative, so as to let the back-end build fully static objects in
more cases.

However, when they cannot be built statically because they contain non-static
parts, aggregates of this sort need always be pre-decomposed.  The change
makes sure this is the case.  But, in order not to go back too far on the
conservative side, it also enhances the circuitry that computes whether an
aggregate can be built statically or not.

The following code must compile quietly:

with Aggr4_Pkg; use Aggr4_Pkg;

package Aggr4 is

   C : constant Rec3 := (Data => (D => One, Value => Zero));

end Aggr4;

package Aggr4_Pkg is

   function F return Integer;

   type Rec1 is tagged record
      I : Integer;
   end record;

   Zero : constant Rec1 := (I => F);

   type Enum is (One, Two);

   type Rec2 (D : Enum := One) is record
      case D is
         when One => Value : Rec1;
         when others => null;
      end case;
   end record;

   type Rec3 is record
      Data : Rec2;
   end record;

end Aggr4_Pkg;          

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

2012-02-08  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_aggr.adb (Compile_Time_Known_Composite_Value):
	New predicate to compute whether a composite value can be
	evaluated at compile time.
	(Component_Not_OK_For_Backend): Use Compile_Time_Known_Value for all
	expressions of elementary type and Compile_Time_Known_Composite_Value
	for all other expressions.
	(Expand_Record_Aggregate): Convert to assignments in the case
	of a type with mutable components if the aggregate cannot be
	built statically.

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