Fix two small but serious bugs in the DWARF location list code that
should have been caught by the automated tests I didn't write.

After emitting debug information for a user variable, mark it as done
so that it doesn't get emitted again. Otherwise it would be written once
per slot it was decomposed into.

Correct a merge error in CL 44350: the location list abbreviations need
to have DW_AT_decl_line too, otherwise the resulting DWARF is gibberish.

Change-Id: I6ab4b8b32b7870981dac80eadf0ebfc4015ccb01
Reviewed-on: https://go-review.googlesource.com/59070
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>