[Ada] Fix bad cases of incorrect modified xref entries

[Arnaud Charlet <charlet at adacore dot com>]

The May_Be_Lvalue routine gave incorrect results in some cases,
notably B appearing in X:=A.B.C; returned true, which is bogus.
This resulted in incorrect 'm' (modified) entries in the xref.

The following is a test program:

package RefMod is
   type Project_Config is record
      F : Integer := 1;
   end record;

   type Project_Data is record
      Config : Project_Config;
   end record;

   procedure Foo;
end RefMod;

package body RefMod is
   procedure Foo is
      P : Project_Data;
      L : Integer;
   begin
      L := P.Config.F;  --  line 6
      P.Config.F := L;  --  line 7
   end Foo;
end RefMod;

If we grep the ali, using grep "*Config" refmod.ali, the output
is a single line:

7r7*Config{2R9} 2|6r14 7m9

Before the patch, this gave 6m14 instead of 6r14

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

2009-04-29  Robert Dewar  <dewar@adacore.com>

	* sem_util.adb (May_Be_Lvalue): Fix cases involving indexed/selected
	components

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