Add "Ada linkage" mode to cooked_index_entry::full_name

Add "Ada linkage" mode to cooked_index_entry::full_name

Unfortunately, due to some details of how the Ada support in gdb
currently works, the DWARF reader will still have to synthesize some
"full name" entries after the cooked index has been constructed.

You can see one particular finding related to this in:

    https://sourceware.org/bugzilla/show_bug.cgi?id=32142

This patch adds a new flag to cooked_index_entry::full_name to enable
the construction of these names.

I hope to redo this part of the Ada support eventually, so that this
code can be removed and the full-name entries simply not created.