Fix -u option in dsymutil, to not emit an extra DW_LNE_set_address if the...

Fix -u option in dsymutil, to not emit an extra DW_LNE_set_address if the original line table was empty

With dsymutil's -u option, only the accelerator tables should be
updated, but with https://reviews.llvm.org/D150554 the -u option will
still re-generate the line table. If the line table was empty, that is,
it was a dummy line table, with no entries in it, dsymutil will always
generate a line table with a DW_LNE_end_sequence, a funky side effect of
this is that when the line table is re-generated, it will always emit a
DW_LNE_set_address first, which will change the line table total size.
This patch addresses this by making sure that if all the line table has
in it is a DW_LNE_end_sequence, it is the same as a dummy entry.

Differential Revision: https://reviews.llvm.org/D151579