Use unique_ptr for objfiles

A while back, I changed objfiles to be held via a shared_ptr.  The
idea at the time was that this was a step toward writing to the index
cache in the background, and this would let gdb keep a reference alive
to do so.  However, since then we've rewritten the DWARF reader, and
the new index can do this without requiring a shared pointer -- in
fact there are patches pending to implement this.

This patch switches objfile management to unique_ptr, which makes more
sense now.

Regression tested on x86-64 Fedora 34.