Ada import functions not in index

Ada import functions not in index

The cooked index does not currently contain entries for Ada import
functions.  This means that whether or not these are visible to
"break" depends on which CUs were previously expanded -- clearly a
bug.

This patch fixes the issue.  I think the comments in the patch explain
the fix reasonably well.

Perhaps one to-do item here is to change GNAT to use
DW_TAG_imported_declaration for these imports.  This may eventually
let us remove some of the current hacks.

This version includes a fix from Simon to initialize the new member.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32511
Acked-By: Simon Marchi <simon.marchi@efficios.com>