Use new to allocate mapped_index

Use new to allocate mapped_index

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.

Tested by the buildbot.

gdb/ChangeLog
2018-05-18  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_per_objfile): Update.
	(struct mapped_index): Add initializers.
	(dwarf2_read_index): Use new.
	(dw2_symtab_iter_init): Update.
	* dwarf2read.h (struct dwarf2_per_objfile) <index_table>: Now a
	unique_ptr.