VFS: Return new file systems as uniquely owned when possible, almost NFC

Uniformly return uniquely-owned filesystems from VFS creation APIs. The
one exception is `getRealFileSystem`, which has a single instance and
needs to be shared.

This is almost NFC, except that it fixes a memory leak in
`vfs::collectVFSFromYAML()`.

Depends on https://reviews.llvm.org/D92888

Differential Revision: https://reviews.llvm.org/D92890