[clangd] Clean the cache of file statuses on vscode-clangd when clangd crashes.

Summary:
Clear the cached file statuses, otherwise we will leave some garbage texts on
the status bar when clangd crashes.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D56540

llvm-svn: 352049