[clang][deps] Cache `VFS::getRealPath()` (#68645)

This PR starts caching calls to
`DependencyScanningWorkerFilesystem::getRealPath()` that we use whenever
we canonicalize module map path. In the case of the real VFS, this
functions performs an expensive syscall that we'd like to do as rarely
as possible.

This PR keeps the real path out of `CachedFileSystemEntry`, since that's
**immutable**; populating the real path on creation of this data
structure (every stat/open) would be expensive.