[llvm] [DirectX] Set whole-module flags prior to evaluating per-function flags (PR #139967)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

I think the bugs you are fixing were mostly found because of the validator? Should we be doing some testing that invokes the validator something similaor to what `llvm/test/tools/dxil-dis/` tests do?

https://github.com/llvm/llvm-project/pull/139967