[clang] Fix handling of adding a file with the same name as an existing dir to VFS (#94461)

When trying to add a file to clang's VFS via `addFile` and a directory
of the same name already exists, we run into a [out-of-bound
access](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/Path.cpp#L244).

The problem is that the file name is [recognised as existing path](
https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L896)
and thus continues to process the next part of the path which doesn't
exist.

This patch adds a check if we have reached the last part of the filename
and return false in that case.
This we reject to add a file if a directory of the same name already
exists.

This is in sync with [this
check](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L903)
that rejects adding a path if a file of the same name already exists.