Fix array stride bug

Fix array stride bug

Investigation of using the Python API with an Ada program showed that
an array of dynamic types was not being handled properly.  I tracked
this down to an oddity of how array strides are handled.

In gdb, an array stride can be attached to the range type, via the
range_bounds object.  However, the stride can also be put into the
array's first field.  From create_range_type_with_stride:

  else if (bit_stride > 0)
    TYPE_FIELD_BITSIZE (result_type, 0) = bit_stride;

It's hard to be sure why this is done, but I would guess a combination
of historical reasons plus a desire (mentioned in a comment somewhere)
to avoid modifying the range type.

This patch fixes the problem by changing type::bit_stride to
understand this convention.  It also fixes one spot that reproduces
this logic.

Regression tested on x86-64 Fedora 32.