Fix "ptype INTERNAL_FUNC" (PR gdb/30105)

Fix "ptype INTERNAL_FUNC" (PR gdb/30105)

Currently, looking at the type of an internal function, like below,
hits an odd error:

 (gdb) ptype $_isvoid
 type = <internal function>type not handled in c_type_print_varspec_prefix()

That is an error thrown from
c-typeprint.c:c_type_print_varspec_prefix, where it reads:

    ...
    case TYPE_CODE_DECFLOAT:
    case TYPE_CODE_FIXED_POINT:
      /* These types need no prefix.  They are listed here so that
	 gcc -Wall will reveal any types that haven't been handled.  */
      break;
    default:
      error (_("type not handled in c_type_print_varspec_prefix()"));
      break;

Internal function types have type code TYPE_CODE_INTERNAL_FUNCTION,
which is not explicitly handled by that switch.

That comment quoted above says that gcc -Wall will reveal any types
that haven't been handled, but that's not actually true, at least with
modern GCCs.  You would need to enable -Wswitch-enum for that, which
we don't.  If I do enable that warning, then I see that we're missing
handling for the following type codes:

   TYPE_CODE_INTERNAL_FUNCTION,
   TYPE_CODE_MODULE,
   TYPE_CODE_NAMELIST,
   TYPE_CODE_XMETHOD

TYPE_CODE_MODULE and TYPE_CODE_NAMELIST and Fortran-specific, so it'd
be a little weird to handle them here.

I tried to reach this code with TYPE_CODE_XMETHOD, but couldn't figure
out how to.  ptype on an xmethod isn't treated specially, it just
complains that the method doesn't exist.  I've extended the
gdb.python/py-xmethods.exp testcase to make sure of that.

My thinking is that whatever type code we add next, the most likely
scenario is that it won't need any special handling, so we'd just be
adding another case to that "do nothing" list.  If we do need special
casing for whatever type code, I think that tests added at the same
time as the feature would uncover it anyhow.  If we do miss adding the
special casing, then it still looks better to me to print the type
somewhat incompletely than to error out and make it harder for users
to debug whatever they need.  So I think that the best thing to do
here is to just remove all those explicit "do nothing" cases, along
with the error default case.

After doing that, I decided to write a testcase that iterates over all
supported languages doing "ptype INTERNAL_FUNC".  That revealed that
Pascal has a similar problem, except the default case hits a
gdb_assert instead of an error:

 (gdb) with language pascal -- ptype $_isvoid
 type =
 ../../src/gdb/p-typeprint.c:268: internal-error: type_print_varspec_prefix: unexpected type
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.

That is fixed by this patch in the same way.

You'll notice that the new testcase special-cases the Ada expected
output:

	} elseif {$lang == "ada"} {
	    gdb_test "ptype \$_isvoid" "<<internal function>>"
	} else {
	    gdb_test "ptype \$_isvoid" "<internal function>"
	}

That will be subject of the following patch.

Approved-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I81aec03523cceb338b5180a0b4c2e4ad26b4c4db
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30105