Split payload of FileName::Real to track both real and virutalized paths.

Split payload of FileName::Real to track both real and virutalized paths.

Such splits arise from metadata refs into libstd.

This way, we can (in a follow on commit) continue to emit the virtual name into
things like the like the StableSourceFileId that ends up in incremetnal build
artifacts, while still using the devirtualized file path when we want to access
the file.

Note that this commit is intended to be a refactoring; the actual fix to the bug
in question is in a follow-on commit.