[clang][deps] Make the service provide the VFS (#181424)
This PR makes it so that the base VFS used during a scan is provided to
the top-level service, not to each worker/tool individually. This
enables resolving a FIXME in the async-scan-modules mode of the scanner,
will clean up [downstream
code](https://github.com/swiftlang/llvm-project/blob/0eb56baa1d92190ed61c1a13caa1112e0f70b109/clang/tools/libclang/CDependencies.cpp#L619-L622),
and will make it more difficult to have mismatching VFSs across workers,
which may cause non-deterministic poisoning of
`DependencyScanningFilesystemSharedCache`.
