C++ PATCH for useless NOTE_INSN_DELETED_DEBUG_LABELs

[Jason Merrill]

This isn't the main problem in debug/58315, but when looking at it I saw 
a bunch of useless

# DEBUG <L0> => NULL

lines, which turned out to be deleted debug labels notes for the 
cdtor_label created in start_preparsed_function.  Since this is an 
internal, unnamed label, we shouldn't have debug information about it, 
but we were forgetting to mark it as artificial.

Tested x86_64-pc-linux-gnu, applying to trunk.
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