`VirtualFile`s are typically used for files with no physical backing.
Use `PhysicalFile` if there's no content provided in the debug info.

Bug: b/148401179
Change-Id: I1e1ed75ce9a9fb0965420267aba7f6d8de958012
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43430Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>