[llvm] [mlir] [NFC][TableGen] Migrate IfDef/Namespace emitter from MLIR to LLVM (PR #161744)

[Rahul Joshi via llvm-commits]

jurahul wrote:

MLIR still uses this code so its tested there, and on LLVM side directive emitter uses the IfDefEmitter, so those unit test exercise it (llvm/test/TableGen/directive*.td)

https://github.com/llvm/llvm-project/pull/161744