[clang][DirectoryWatcher][NFC] Swapping asserts for llvm fatal_error in create

I also have replaced all the instances of
"auto DW = DirectoryWatcher::create" with
llvm::Expected<std::unique_ptr<DirectoryWatcher>> DW = DirectoryWatcher::create
to make it more clear that DirectoryWatcher::create is returning an Expected.

I've also allowed for logAllUnhandledErrors to consume errors in the case were
DirectoryWatcher::create produces them.

Differential Revision: https://reviews.llvm.org/D65829

llvm-svn: 368108