[dotest] Add --dwarf-version to override the tested DWARF version.

On the matrix bot on GreenDragon [1] we want to run the test suite
against different DWARF versions. The idea here is not to replace
targeted tests for certain DWARF features, but rather to provide an easy
way to support this configuration.

[1] http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-matrix/

Differential revision: https://reviews.llvm.org/D66370

llvm-svn: 369272