gdb/python: add Type.is_signed property

Add a new read-only property, Type.is_signed, which is True for signed
types, and False otherwise.

This property should only be read on types for which Type.is_scalar is
true, attempting to read this property for non-scalar types will raise
a ValueError.

I chose 'is_signed' rather than 'is_unsigned' in order to match the
existing Architecture.integer_type method, which takes a 'signed'
parameter.  As far as I could find, that was the only existing
signed/unsigned selector in the Python API, so it seemed reasonable to
stay consistent.

gdb/python/py-type.c

@@ -446,6 +446,27 @@ typy_is_scalar (PyObject *self, void *closure)
     Py_RETURN_FALSE;
 }
 
+/* Return true if this type is signed.  Raises a ValueError if this type
+   is not a scalar type.  */
+
+static PyObject *
+typy_is_signed (PyObject *self, void *closure)
+{
+  struct type *type = ((type_object *) self)->type;
+
+  if (!is_scalar_type (type))
+    {
+      PyErr_SetString (PyExc_ValueError,
+		       _("Type must be a scalar type"));
+      return nullptr;
+    }
+
+  if (type->is_unsigned ())
+    Py_RETURN_FALSE;
+  else
+    Py_RETURN_TRUE;
+}
+
 /* Return the type, stripped of typedefs. */
 static PyObject *
 typy_strip_typedefs (PyObject *self, PyObject *args)
@@ -1502,6 +1523,8 @@ static gdb_PyGetSetDef type_object_getset[] =
     "The objfile this type was defined in, or None.", NULL },
   { "is_scalar", typy_is_scalar, nullptr,
     "Is this a scalar type?", nullptr },
+  { "is_signed", typy_is_signed, nullptr,
+    "Is this an signed type?", nullptr },
   { NULL }
 };