[clang] [llvm] [HLSL][DirectX][SPIRV] Implement the `fma` API (PR #185304)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

This pr is getting into a much better spot, please dont' forget we need test cases as part of completion of this ticket. https://github.com/llvm/offload-test-suite/issues/916

https://github.com/llvm/llvm-project/pull/185304