Correctly handle forward DIE references in scanner

The cooked index scanner has special code to handle forward DIE
references.  However, a bug report lead to the discovery that this
code does not work -- the "deferred_entry::spec_offset" field is
written to but never used, i.e., the lookup is done using the wrong
key.

This patch fixes the bug and adds a regression test.

The test in the bug itself used a thread_local variable, which
provoked a failure at runtime.  This test instead uses "maint print
objfiles" and then inspects to ensure that the entry in question has a
parent.  This lets us avoid a clang dependency in the test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30271