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.

gdb/gdbtypes.c

@@ -1280,13 +1280,7 @@ update_static_array_size (struct type *type)
       int stride;
       struct type *element_type;
 
-      /* If the array itself doesn't provide a stride value then take
-	 whatever stride the range provides.  Don't update BIT_STRIDE as
-	 we don't want to place the stride value from the range into this
-	 arrays bit size field.  */
-      stride = TYPE_FIELD_BITSIZE (type, 0);
-      if (stride == 0)
-	stride = range_type->bit_stride ();
+      stride = type->bit_stride ();
 
       if (!get_discrete_bounds (range_type, &low_bound, &high_bound))
 	low_bound = high_bound = 0;