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

[Damyan Pepper via llvm-commits]

https://github.com/damyanp commented:

I have a general concern around why everything before this code allows there to be multiple entry points, but now this is the place which starts to enforce that there's only one.

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