[clangd] Fix remote index build for macOS with Homebrew-installed gRPC and Protobuf

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79387