[clang] [llvm] [NFC][HLSL] Move emitter out of AMDGPU.cpp (PR #133251)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> Thanks for creating this PR, LGTM (I wasn't able to do it earlier as one of my kids was ill, so away from laptop)

No worries. Thanks for the approval!

https://github.com/llvm/llvm-project/pull/133251