[clangd] Update the out-of-date yaml-symbol-file flag in clangd.

Summary:
The flag is stale due to the recent changes of clangd indexer, this
patch renames the flag to "index-file".

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D52976

llvm-svn: 343963