Debug Info: support for gdwarf-2 gdwarf-3 gdwarf-4

These options will add a module flag with name "Dwarf Version".
The behavior flag is currently set to Warning, so when two values disagree,
a warning will be emitted.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184276 91177308-0d34-0410-b5e6-96231b3b80d8