Fix use-after-free when destroying objfile

Fix use-after-free when destroying objfile

The recent patch to heap-allocate compunit_symtabs introduced a
use-after-free that can occur when destroying an objfile.  The bug
here is that the objfile obstack is destroyed before compunit_symtabs;
but the compunit_symtabs destructor refers to the symtabs, which are
allocated on the obstack.

This patch fixes the problem.  This was reported using ASAN, but I
reproduced it with valgrind and verified that this fixes the problem.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33435