[clangd] Strip plugin arguments in clangd-indexer.

Summary: This would allow clangd-indexer runs on chromium repo.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59022

llvm-svn: 355599