[clang] Exclude RedirectingFileSystem with null OverlayFileDir in VFSUsage (PR #128267)

[Hiroshi Yamauchi via cfe-commits]

hjyamauchi wrote:

An assertion failure like the above was encountered in the swift toolchain in the context of this PR https://github.com/swiftlang/swift/pull/78184 which uses a module map VFS overlay on top of the read-only Windows MSVC/ucrt include paths.

https://github.com/llvm/llvm-project/pull/128267