[clangd] Load static index asynchronously, add tracing.

Summary:
Like D51475 but simplified based on recent patches.
While here, clarify that loadIndex() takes a filename, not file content.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D51638

llvm-svn: 341376