[clang] [llvm] Adding splitdouble HLSL function (PR #109331)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

PR looks good. CGBuiltin.cpp were minor fix requests.  If we are going to revisit the DXILOpLowering.cpp I'd like to see an issue number to track the work.

https://github.com/llvm/llvm-project/pull/109331