Update DW_TAG_subprogram parsing for clang

Update DW_TAG_subprogram parsing for clang

Clang doesn't give a name for some artificial subprograms. In that
case just use "<artificial>" as the name of the DW_TAG_subprogram.

Clang also sometimes generates a DW_TAG_subprogram without any
attributes. These aren't really useful for us. So just silently skip
them.

If we warn about subprograms without a name, specification or abstract
origin, also emit the index in the .debug_info section to make it
easier to look them up.