[FileSystemStatCache] Return std::error_code from stat cache methods

Summary:
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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60735