[Ada] Disable name generation for External_Tag and Expanded_Name

[Ada] Disable name generation for External_Tag and Expanded_Name

In order to avoid exposing internal names of tagged types in the
binary code generated by the compiler this enhancement facilitates
initializes the External_Tag of a tagged type with an empty string
when pragma No_Tagged_Streams is applicable to the tagged type, and
facilitates initializes its Expanded_Name with an empty string when
pragma Discard_Names is applicable to the tagged type.

This enhancement can be verified by means of the following small
test:

package Library_Level_Test is
   type Typ_01 is tagged null record;    --  Case 1: No pragmas

   type Typ_02 is tagged null record;    --  Case 2: Discard_Names
   pragma Discard_Names (Typ_02);

   pragma No_Tagged_Streams;
   type Typ_03 is tagged null record;    --  Case 3: No_Tagged_Streams

   type Typ_04 is tagged null record;    --  Case 4: Both pragmas
   pragma Discard_Names (Typ_04);
end;

Commands:
  gcc -c -gnatD library_level_test.ads
  grep "\.TYP_" library_level_test.ads.dg

Output:
     "LIBRARY_LEVEL_TEST.TYP_01["00"]";
     "LIBRARY_LEVEL_TEST.TYP_02["00"]";
     "LIBRARY_LEVEL_TEST.TYP_03["00"]";

2018-05-22  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_disp.adb (Make_DT): Initialize the External_Tag with an empty
	string when pragma No_Tagged_Streams is applicable to the tagged type,
	and initialize the Expanded_Name with an empty string when pragma
	Discard_Names is applicable to the tagged type.

From-SVN: r260528