gdb/dwarf: allocate DWP dwarf2_section_info with new

gdb/dwarf: allocate DWP dwarf2_section_info with new

For the same reason as explained in the previous patch (allocations on
obstacks aren't thread-safe), change the allocation of
dwarf2_section_info object for dwo files within dwp files to use "new".

The dwo_file::section object is not always owned by the dwo_file, so
introduce a new "dwo_file::section_holder" object that is only set when
the dwo_file owns the dwarf2_section_info.

Change-Id: I74c4608573c7a435bf3dadb83f96a805d21798a2
Approved-By: Tom Tromey <tom@tromey.com>