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.

gdb/objfiles.h

@@ -409,7 +409,7 @@ public:
      remove it from the program space's list.  In some cases, you may
      need to hold a reference to an objfile that is independent of its
      existence on the program space's list; for this case, the
-     destructor must be public so that shared_ptr can reference
+     destructor must be public so that unique_ptr can reference
      it.  */
   ~objfile ();