[clang] Adopt llvm::ErrorOr in FileManager methods

Previously, the FileManager would use NULL returns to signify whether a file existed, but that doesn’t cover permissions issues or anything else that might occur while trying to stat or read a file. Instead, convert getFile and getDirectory into returning llvm::ErrorOr

Signed-off-by: Harlan Haskins <harlan@apple.com>
llvm-svn: 367615