[clang] [llvm] [llvm][clang] Propagate VFS through LTO to `PGOOptions` (PR #159671)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Yes, `clang/lib/CodeGen/BackendUtil.cpp` does pass the `CompilerInstance`'s VFS into `thinBackend()`, so that `runNewPMPasses()` in `llvm/lib/LTO/LTOBackend.cpp` doesn't need to call `vfs::getRealFileSystem()`

https://github.com/llvm/llvm-project/pull/159671