gdb/testsuite: add test for negative subrange bounds with unsigned form

gdb/testsuite: add test for negative subrange bounds with unsigned form

I am looking at this code [1]:

  /* Normally, the DWARF producers are expected to use a signed
     constant form (Eg. DW_FORM_sdata) to express negative bounds.
     But this is unfortunately not always the case, as witnessed
     with GCC, for instance, where the ambiguous DW_FORM_dataN form
     is used instead.  To work around that ambiguity, we treat
     the bounds as signed, and thus sign-extend their values, when
     the base type is signed.  */
  negative_mask =
    -((ULONGEST) 1 << (base_type->length () * TARGET_CHAR_BIT - 1));
  if (low.kind () == PROP_CONST
      && !base_type->is_unsigned () && (low.const_val () & negative_mask))
    low.set_const_val (low.const_val () | negative_mask);
  if (high.kind () == PROP_CONST
      && !base_type->is_unsigned () && (high.const_val () & negative_mask))
    high.set_const_val (high.const_val () | negative_mask);

Nothing in the testsuite seems to exercise it, as when I remove it, all
of gdb.dwarf2 still passes.  And tests in other directories would be
compiler-dependent, so would rely on having a buggy compiler.

Update gdb.dwarf2/subrange.exp to have a test for it.  When removing the
code above, the new test fails with:

  ptype array_with_buggy_negative_bounds_type^M
  type = array [240..244] of signed_byte^M
  (gdb) FAIL: gdb.dwarf2/subrange.exp: ptype array_with_buggy_negative_bounds_type

instead of the expected:

  ptype array_with_buggy_negative_bounds_type^M
  type = array [-16..-12] of signed_byte^M
  (gdb) PASS: gdb.dwarf2/subrange.exp: ptype array_with_buggy_negative_bounds_type

[1] https://gitlab.com/gnutools/binutils-gdb/-/blob/5ea14aa4e53fa37f4ba4517497ed2c1e4c60dee2/gdb/dwarf2/read.c#L17681-17695

Change-Id: I1992a3ff0cb1e90fa8a9114dae6c591792f059c2