[clangd] Change diskbackedstorage to be atomic

Summary:
There was a chance that multiple clangd instances could try to write
same shard, in which case we would get a malformed file most likely. This patch
changes the writing mechanism to first write to a temporary file and then rename
it to fit real destination. Which is guaranteed to be atomic by POSIX.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D55417

llvm-svn: 349348