[DirectX] Fix build break caused by bitcode writer changes (#133770)
commit:
https://github.com/llvm/llvm-project/commit/68947342b75cc71f3ac9041d11db086d8d074336
added a new `writeDIFixedPointType` function.
However, `DIFixedPointType` is not supported in DXIL so we need to add a
fail fast case for this to fix the build.
this change fixes a build break introduced by
https://github.com/llvm/llvm-project/pull/129596