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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

It is important that gets mentioned because this doesn't affect LTO builds that triggered by linker. The `-fthinlto-index` is only used by distributed builds like Bazel for distributed thinLTO so it is important to mention what changed.

You can totally construct a test case that `-fthinlto-index` also passed with a vfs overlay to the clang. That should not go to real FS

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