[FileSystemStatCache] Return std::error_code from stat cache methods
Previously, we would return true/false signifying if the cache/lookup
succeeded or failed. Instead, provide clients with the underlying error
that was thrown while attempting to look up in the cache.
Since clang::FileManager doesn't make use of this information, it discards the
error that's received and casts away to bool.
This change is NFC.
Reviewers: benlangmuir, arphaman
Subscribers: dexonsmith, cfe-commits
Differential Revision: https://reviews.llvm.org/D60735