Graphics tests fail on Clang with d3d12

[Alexander-Johnston]

Previously the graphics tests were unsupported on Clang due to SV_TARGET not having being implemented.
Now that it is implemented they pass on Vulkan (tested locally on Windows and Ubutnu 24.04 LTS with an AMD GPU), but fail on Windows targeting d3d12.

Each test errors during with

# .---command stderr------------
# | error: Unsupported intrinsic llvm.dx.load.input.v4f32 for DXIL lowering
# | error: Unsupported intrinsic llvm.dx.store.output.v4f32 for DXIL lowering
# | 2 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

Testing SimpleTriangle.test specifically it appears this occurs with both the vertex and pixel shaders.

Tested with clang commit 25b6a15dfd228a4bf10c77240cecb26864e0e527

[farzonl]

This is because these intrinsics have not been implemented in the backend yet. They were most recently added for the semantics pr: llvm/llvm-project@b8a814e#diff-bd26b60402c9e3e0f47a017096fe53a506176f4f21ff980433cabdc228c4dddb

@Keenuts might be able to give more insight on when the lowering for DirectX will happen.

As a side note this also looks like it needs to go through scalarization.

[damyanp]

Next steps for this will be to file tasks to implement the missing functionality, and then this bug can be dupe'd against those tasks.

It turns out we have existing issues for fix: llvm/wg-hlsl#145 and llvm/wg-hlsl#146. Please use these issues in the XFAIL comment.

[Keenuts]

For context: those are mostly placeholder intrinsics for the DXIL backend. How to go from LLVM-IR to DXIL is not completely designed yet, key components like shader signature is not yet finalized, so I emitted those as some kind of placeholder but don't expect this to work for DirectX.