Use std::unique_ptr in dwarf2_read_debug_names

Use std::unique_ptr in dwarf2_read_debug_names

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.

Tested by the buildbot.

ChangeLog
2018-05-21  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_read_debug_names): Use std::unique_ptr.
	(mapped_index_base): Use DISABLE_COPY_AND_ASSIGN.  Default
	constructor.