[clangd] Store index in '.clangd/index' instead of '.clangd-index'

Summary: To take up the .clangd folder for other potential uses in the future.

Reviewers: kadircet, sammccall

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58440

llvm-svn: 354505