[RFA] Use std::unique_ptr in dwarf2_read_debug_names

[Simon Marchi]

On 2018-05-19 11:23 AM, Tom Tromey wrote:
> This changes dwarf2_read_debug_names to use std::unique_ptr from the
> outset.  This simplifies the code that installs the resulting map into
> dwarf2_per_objfile.

LGTM.

We maybe could add DISABLE_COPY_AND_ASSIGN to mapped_index_base, to ensure
we don't make copies of mapped_index/mapped_debug_names by mistake.

Simon