Remove LLVM_CONFIG configuration variable

[TravisCardwell]

This commit removes the LLVM_CONFIG configuration variable. Users should configure PATH so that the desired llvm-config is found. The motivation for this change is that we also use the clang executable. It is expected behavior to find clang via PATH or ${LLVM_PATH}/bin, while finding it via LLVM_CONFIG is potentially unexpected.

The logic is simplified. LLVM_PATH is used if set. Otherwise, llvm-config is used if found. There is a configuration error if LLVM_PATH is not set and llvm-config is not found.

If users do not want to use LLVM_PATH, then it should not be set. Note that the KyleMayes/install-llvm-action action sets this environment variable. (We will likely stop using that action in the future so that we can test new versions soon after they are released.)

[edsko]

@bolt12 Can you verify if LLVM_PATH is usable on nix? Concretely, if all folders you need for llvm (bin, lib, include) are under one parent folder, then you can use LLVM_PATH rather than writing an llvm-config-shim, might simplify that slightly (for your cross compilation example). If not, you'll need to make sure use PATH rather than LLVM_CONFIG to point to your shim (perhaps you already are, I don't remember).