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

[Rahul Joshi via llvm-commits]

jurahul wrote:

Ok, I adopted the NamespaceEmitter in once place, so we now have lit tests in TableGen that exercise both classes.

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