[DirectX backend] Illegal Dwarf Version emitted to final dxil.

[python3kgae]

Module flag !{i32 7, !"Dwarf Version", i32 5} could be send to DirectX backend in Module Flags Metadata.

There're 2 things DXIL not supported due to DXIL is based on llvm 3.7.

1. The behavior flag 7 (Max).
   llvm 3.7 only support behavior flags up to 6.
   Set it to 7 will hit error when create llvm 3.7 module.
2. Dwarf Version 5.
   llvm 3.7 only support Dwarf Version up to 4.

For behavior flag, clang 3.7 will set it to 2 (Warning) for "Dwarf Version".
The fix could be to do the same thing when targeting DXIL.

The Dwarf Version issue could be fixed by force the CodeGenOpts.DwarfVersion to 4 when targeting DXIL.