[NFC] [DirectX] Prefix for intrinsics should be dx

`dxil` is an architecture supported by the DirectX backend. These
intrinsics will likely be shared with other DirectX architectures like
`dxbc`. Using a common prefix `dx` will make it more intuitive.

Also the `dx` prefix is already set in the Triple, which causes
intrinsics described here to be unmatchable via the ClangBuiltin
mechanism.