gdb: make type::bounds work for array and string types
Getting the bounds of an array (or string) type is a common operation,
and is currently done through its index type:
my_array_type->index_type ()->bounds ()
I think it would make sense to let the `type::bounds` methods work for
arrays and strings, as a shorthand for this. It's natural that when
asking for the bounds of an array, we get the bounds of the range type
used as its index type. In a way, it's equivalent as the now-removed
TYPE_ARRAY_{LOWER,UPPER}_BOUND_IS_UNDEFINED and
TYPE_ARRAY_{LOWER,UPPER}_BOUND_VALUE, except it returns the
`range_bounds` object. The caller is then responsible for getting the
property it needs in it.
I updated all the spots I could find that could take advantage of this.
Note that this also makes `type::bit_stride` work on array types, since
`type::bit_stride` uses `type::bounds`. `my_array_type->bit_stride ()`
now returns the bit stride of the array's index type. So some spots
are also changed to take advantage of this.