[llvm] [MLIR] Fix the MLIR build by properly adding external directory- (PR #222749)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

I'm not sure I fully understand how it broke, can you ask the agent to figure out which patch broke it and provide us with an analysis?

https://github.com/llvm/llvm-project/pull/222749