[clangd] Add a Filesystem that overlays Dirty files.

Create a `ThreadsafeFS` in the `DraftStore` that overlays the dirty file contents over another `ThreadsafeFS`.
This provides a nice thread-safe interface for using dirty file contents throughout the codebase, for example cross file refactoring.
Creating a Filesystem view will overlay a snapshot of the current contents, so if the draft store is updated while the view is being used, it will contain stale contents.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D94554