De-duplicate .gdb_index

De-duplicate .gdb_index

This de-duplicates variables and types in .gdb_index, making the new
index closer to what gdb generated before the new DWARF scanner
series.  Spot-checking the resulting index for gdb itself, it seems
that the new scanner picks up some extra symbols not detected by the
old one.  I tested both the new and old versions of gdb on both new
and old versions of the index, and startup time in all cases is
roughly the same (it's worth noting that, for gdb itself, the index no
longer provides any benefit over the DWARF scanner).  So, I think this
fixes the size issue with the new index writer.

Regression tested on x86-64 Fedora 34.