The other answer did not work for me, but what did work for me on M1:

brew install llvm

export LLVM_CONFIG_PATH=$(brew --prefix llvm)/bin/llvm-config
export LIBCLANG_PATH=$(brew --prefix llvm)/lib

echo 'export LLVM_CONFIG_PATH=$(brew --prefix llvm)/bin/llvm-config' >> ~/.zshrc
echo 'export LIBCLANG_PATH=$(brew --prefix llvm)/lib' >> ~/.zshrc

source ~/.zshrc

cargo clean and cargo build afterwards:)