[APINotes] Adopt FileManager's error-returning APIs
[APINotes] Adopt FileManager's error-returning APIs

In https://reviews.llvm.org/D65534, FileManager's getFile and
getDirectory APIs were updated to return errors, not just null-or-valid
pointers. Update our internal APINotes usage of FileManager for this.

Fixes rdar://53837986

(cherry picked from commit 1c2ca31a8c6153000daaa40120ec021fa62f1077)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang