LLVM <= 4.0 used a non-standard interpretation of `DW_OP_plus`.  In the
DWARF standard, this adds two items on the expressions stack.  LLVM's
behavior was more like DWARF's `DW_OP_plus_uconst` -- adding a constant
that follows the op.  The patch series starting with [D33892] switched
to the standard DWARF interpretation, so we need to follow.

[D33892]: https://reviews.llvm.org/D33892