Don't add type linkage names to cooked index

Don't add type linkage names to cooked index

The compiler will sometimes emit a linkage name for a type, like:

    <1d3>   DW_AT_linkage_name: (indirect string, offset: 0x106f): 11__mbstate_t

These names aren't very useful, and this patch changes the DWARF
reader so that they are ignored by the cooked index.