[clang] [clang][HLSL] Move hlsl_wave_get_lane_index to EmitHLSLBuiltinExpr (PR #87131)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

minor nit pick, you don't have to do this one: could you make the HLSL prefix first. It helps with some bookeeping we are doing.

https://github.com/llvm/llvm-project/pull/87131