[Ada] problem printing negative integer values in packed arrays.

[Ada] problem printing negative integer values in packed arrays.

Consider the following declarations:

   type Signed_Small is new Integer range - (2 ** 5) .. (2 ** 5 - 1);
   type Signed_Simple_Array is array (1 .. 4) of Signed_Small;
   pragma Pack (Signed_Simple_Array);
   SSA : Signed_Simple_Array := (-1, 2, -3, 4);

GDB currently print its value incorrectly for the elements that
are negative:

    (gdb) print ssa
    $1 = (65535, 2, 1048573, 4)
    (gdb) print ssa(1)
    $2 = 65535
    (gdb) print ssa(2)
    $3 = 2
    (gdb) print ssa(3)
    $4 = 1048573
    (gdb) print ssa(4)
    $5 = 4

What happens is that the sign-extension is not working because
we're trying to do left shift with a negative count. In
ada_value_primitive_packed_val, we have a loop which populates
the extra bits of the target (unpacked) value, after extraction
of the data from the original (packed) value:

        while (ntarg > 0)
          {
            accum |= sign << accumSize;
            unpacked[targ] = accum & ~(~0L << HOST_CHAR_BIT);
!!! ->      accumSize -= HOST_CHAR_BIT;
            accum >>= HOST_CHAR_BIT;
            ntarg -= 1;
            targ += delta;
          }

At each iteration, accumSize gets decremented by HOST_CHAR_BIT,
which can easily cause it to become negative, particularly on
little endian targets, where accumSize is at most HOST_CHAR_BIT - 1.
This causes us to perform a left-shift operation with a negative
accumSize at the next loop iteration, which is undefined, and
acutally does not produce the effect we wanted (value left untouched)
when the code is compiled with GCC.

This patch fixes the issue by simply setting accumSize to zero
if negative.

gdb/ChangeLog:

        * ada-lang.c (ada_value_primitive_packed_val): Make sure
        accumSize is never negative.

gdb/testsuite/ChangeLog:

        * gdb.ada/pckd_neg: New testcase.