…forms.

Line 0 has a special meaning in DWARF. From the version 5 spec:

    The compiler may emit the value 0 in cases
    where an instruction cannot be attributed to any
    source line.

DUMMY_SP spans cannot be attributed to any line. However, because rustc
internally stores line numbers starting at zero, lookup_debug_loc() adjusts
every line number by one. Special casing DUMMY_SP to actually emit line 0
ensures rustc communicates to the debugger that there's no meaningful source
code for this instruction, rather than telling the debugger to jump to line 1
randomly.