Add appropriate LLVM module flags for debug info.

Set "Dwarf Version" to 2 on OS X to avoid toolchain incompatibility, and
set "Debug Info Version" to prevent debug info from being stripped from
bitcode.

Fixes #11352.
auto merge of #11864 : comex/rust/11352, r=alexcrichton

Set "Dwarf Version" to 2 on OS X to avoid toolchain incompatibility, and
set "Debug Info Version" to prevent debug info from being stripped from
bitcode.

Fixes #11352.