[RFA] Use new to allocate mapped_index

[Simon Marchi]

On 2018-05-18 11:37, Tom Tromey wrote:
> This changes struct mapped_index to be allocated with new.  This
> simplifies the creation a bit (see dwarf2_read_index) and also removes
> a somewhat ugly explicit destructor call from ~dwarf2_per_objfile.

LGTM, thanks.

Simon