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

[Jan Svoboda via llvm-commits]

jansvoboda11 wrote:

Yes, this only kicks in when Clang gets invoked with the `-fthinlto-index=` argument, but it doesn't affect how that particular file gets loaded. It only affects the files passed via `PGOOptions`, like `-fprofile-instrument-use-path=` and `-fprofile-sample-use=`. So I think spelling out `-fthinlto-index=` in the message might be a bit misleading, since that file still goes straight to the real FS.

Also, I'm not sure how necessary it is to test this specifically. The idea is that once all such uses of the real FS are abstracted away, we turn on sandboxing in asserts builds (that verifies the one true VFS is being used everywhere), and we get test coverage that way.

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