[Ada] Crash with references to GNAT packed arrays handling

Consider the following declarations:

  type Packed_Array is array (Natural range <>) of Boolean;
  pragma Pack (Packed_Array);

  function Make (H, L : Natural) return Packed_Array is
  begin
     return (H .. L => False);
  end Make;

  A1 : Packed_Array := Make (1, 2);
  A2 : Packed_Array renames A1;

One possible DWARF translation for A2 is:

  <3><1e4>: Abbrev Number: 21 (DW_TAG_variable)
     <1e5>   DW_AT_name                 : a2
     <1ea>   DW_AT_type                 : <0x1d9>

  <3><1d9>: Abbrev Number: 22 (DW_TAG_const_type)
     <1da>   DW_AT_type                 : <0x1de>
  <3><1de>: Abbrev Number: 23 (DW_TAG_reference_type)
     <1e0>   DW_AT_type                 : <0x1a3>
  <3><1a3>: Abbrev Number: 17 (DW_TAG_array_type)
     <1a4>   DW_AT_name                 : foo__Ta1S___XP1
     <1a8>   DW_AT_GNAT_descriptive_type: <0x16b>

  <3><16b>: Abbrev Number: 6 (DW_TAG_typedef)
     <16c>   DW_AT_name                 : foo__Ta1S
     <172>   DW_AT_type                 : <0x176>
  <3><176>: Abbrev Number: 17 (DW_TAG_array_type)
     <177>   DW_AT_name                 : foo__Ta1S
     <17b>   DW_AT_GNAT_descriptive_type: <0x223>

Here, foo__Ta1S___XP1 is the type used for the code generation while
foo__Ta1S is the source-level type. Both form a valid GNAT encoding for
a packed array type.

Trying to print A2 (1) can make GDB crash. This is because A2 is defined
as a reference to a GNAT encoding for a packed array. When decoding
constrained packed arrays, the ada_coerce_ref subprogram follows
references and returns a fixed type from the target type, peeling
the GNAT encoding for packed arrays. The remaining code assumes that
the resulting type is still such an encoding while we only have
a standard GDB array type, hence the crash:

  arr = ada_coerce_ref (arr);
  [...]
  type = decode_constrained_packed_array_type (value_type (arr));

decode_constrained_packed_array_type assumes that its argument is
such an encoding. From its front comment:

  /* The array type encoded by TYPE, where
     ada_is_constrained_packed_array_type (TYPE).  */

This patch simply replaces the call to ada_coerce_ref with a call
to coerce_ref in order to avoid prematurely transforming
the packed array type as a side-effect. This way, the remaining code
will always work with a GNAT encoding.

gdb/ChangeLog:

	* ada-lang.c (decode_constrained_packed_array): Perform a
	minimal coercion for reference with coerce_ref instead of
	ada_coerce_ref.
[testsuite/Ada] New testcase for packed array renaming.

gdb/testsuite/ChangeLog:

        * gdb.ada/pckd_arr_ren: New testcase.