[llvm] [DXIL] Add support for root signature flag element in DXContainer (PR #123147)

[Damyan Pepper via llvm-commits]

https://github.com/damyanp approved this pull request.

LGTM - please try and get approval from the other reviewers before completing.

@bogner - wanted to check the "function gets pruned leaving behind some null metadata" thing is not something to worry about.

https://github.com/llvm/llvm-project/pull/123147