Debug Info: use module flag to set up Dwarf version.

Correctly handles ref_addr depending on the Dwarf version. Emit Dwarf with
version from module flag.

TODO: turn on/off features depending on the Dwarf version.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185484 91177308-0d34-0410-b5e6-96231b3b80d8