[clangd] Log a message when gRPC support is off, but remote-index is configured

Before this change clangd would emit a diagnostic whenever remote-index
was configured but binary didn't have grpc support.

This can be annoying when projects are configuring remote-index through their
configs but developers have a clangd binary without the support.

Differential Revision: https://reviews.llvm.org/D100103

(cherry picked from commit b9b708ee)