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

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

Ah, I think it only applies to `clang -fthinlto-index=` invocation, not LTO/PGOOptions. Can you adjust the commit message and also add a test for it?

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