Use llvm-raw override for build w/ local LLVM repo
Michael, I think you're a better reviewer for this (or able to find one) than me. Thanks Mihai, It looks fine to me but let me ask someone working closer on the mlir side to take a look.  Geoffrey i was pointed your way as someone who would be good to take a look if you don't mind.  Looks like there's a typo in L33 of this PR: `LLVM_BAZEL_OVERLAY` should be `LLVM_SRC` as overlay is not used here. (Sorry for the late comments, I had missed the submit comment button)
Building with a llvm-project override to use a local LLVM repo as instructed by tensorflow/compiler/mlir/README.md fails due to missing vars.bzl. Instead of adding one more step to create this file in the Readme, this commit instead instruct to override llvm-raw target. This let llvm_configure() set up the bazel overlay and set it up and also saves on pointless LLVM download.

Fixes #59739 