[clangd] Fix remote index build on macOS

macOS builds suddenly started failing:

https://github.com/kirillbobyrev/indexing-tools/runs/925090879

This patch makes use of imported libraries and fixes builds for macOS.

Landing this without a review since the patch is quite straightforward
and I've been testing it on my local macOS machine for a while.

Differential Revision: https://reviews.llvm.org/D84928