[C++ Patch] Improve compute_array_index_type locations

[Jason Merrill]

On 11/6/18 4:01 AM, Paolo Carlini wrote:
> when I improved create_array_type_for_decl I didn't notice that it calls 
> compute_array_index_type as helper, which simply needs to have the 
> location information propagated. Tested x86_64-linux.

This looks like it will use the declarator-id location in diagnostics 
about the array bound expression, which isn't optimal; if 'size' has a 
location, we should use that instead.

Jason