[clangd] Allow to use vfs::FileSystem for file accesses.

Summary:
Custom vfs::FileSystem is currently used for unit tests.
This revision depends on https://reviews.llvm.org/D33397.

Reviewers: bkramer, krasimir

Reviewed By: bkramer, krasimir

Subscribers: klimek, cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D33416

llvm-svn: 303977