[clangd] Update the out-of-date yaml-symbol-file flag in clangd.
[clangd] Update the out-of-date yaml-symbol-file flag in clangd.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

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@343963 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra