[clangd] Mark results of clangd requests with a tag provided by the FileSystemProvider.

Summary:
This allows an implementation of FileSystemProvider that can track which vfs::FileSystem
were used for each of the requests.

Reviewers: bkramer, krasimir

Reviewed By: bkramer

Subscribers: klimek, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D33678

llvm-svn: 304214