[llvm] [LLVM][TableGen] Move DecoderEmitter output to anonymous namespace (PR #136214)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Thanks, FYI this also fixes the regression that was introduced by my earlier PR to adjust the decoder table `NumToSkip` per backend: https://github.com/llvm/llvm-project/pull/136019#issuecomment-2813657821

https://github.com/llvm/llvm-project/pull/136214