[DirectX] Update DXContainerGlobals to get the shader flags from metadata rather than analysis

[bogner]

DXContainerGlobals currently grabs shader flags from the analysis. This seems reasonable at a high level, but there are a couple of caveats:

1. We'd like to disallow running shader flags analysis after op lowering (See [DirectX] Run shader flags analysis before DXILOpLowering #120119), which means this would need to rely on the flags analysis being preserved since then
2. The flags are already embedded in DXIL metadata at this point, so we should just be able to copy them from there.

We should update the pass to simply pull the flags in an opaque way from metadata and put them into the appropriate container part.

[bogner]

Had a half baked thought worth considering here - what if we continue to use the shader flags analysis in DXContainerGlobals, but we teach the analysis to trust the metadata? That is to say, the analysis can just fill the flags structure if the metadata is present, and since the metadata is always present post-lowering, then it can run at any time. This has the interesting effect that the flags pass can be run on DXIL and get the flags from metadata if we at some point want to reuse the pass in the DXIL->modern LLVM path.