[Ada] Minor tweak to output of -gnatR

[Ada] Minor tweak to output of -gnatR

This changes the output of -gnatR for extensions of tagged record types to
avoid displaying the internal _Parent component, which overlaps with other
components and is thus more confusing than helpful.

For the following hierarchy:

  type R1 is tagged record
    I : Integer;
  end record;

  type R2 is new R1 with record
    C : Character;
  end record;

the output -gnatR must now be:

for R1'Object_Size use 128;
for R1'Value_Size use 96;
for R1'Alignment use 8;
for R1 use record
   _Tag at 0 range  0 .. 63;
   I    at 8 range  0 .. 31;
end record;

for R2'Object_Size use 192;
for R2'Value_Size use 136;
for R2'Alignment use 8;
for R2 use record
   _Tag at  0 range  0 .. 63;
   I    at  8 range  0 .. 31;
   C    at 16 range  0 ..  7;
end record;

2018-05-28  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* repinfo.adb (Compute_Max_Length): Skip _Parent component.
	(List_Record_Layout): Likewise.

From-SVN: r260819