[DirectX backend] Add pass to emit dxil metadata.

A new pass DxilEmitMetadata is added to translate information saved in llvm ir into metadata to match DXIL spec.

Only generate DXIL validator version in this PR.

In llvm ir, validator version is saved in ModuleFlag with "dx.valver" as Key.

  !llvm.module.flags = !{!0, !1}
  !1 = !{i32 6, !"dx.valver", !2}
  !2 = !{i32 1, i32 1}

DXIL validator version has major and minor versions that are specified as named metadata:

  !dx.valver = !{!2}
  !2 = !{i32 1, i32 7}

Reviewed By: kuhar, beanz

Differential Revision: https://reviews.llvm.org/D125158