DWARF discriminators are used to distinguish multiple control flow paths
on the same source location. When this happens, instructions across
basic block boundaries will share the same debug location.
This pass detects this situation and creates a new lexical scope to
one of the two instructions. This lexical scope is a child scope of
the original and contains a new discriminator value. This discriminator
is then picked up from MCObjectStreamer::EmitDwarfLocDirective to be
written on the object file.
Finally, the discriminator is emitted in EmitDwarfLineTable according
to the DWARF spec.