clang-format: Assert in-memory file created in createInMemoryFile, NFC

`SourceManager::createFileID` asserts that the given `FileEntry` is not
null, so remove the logic that passed in `nullptr`. Since we just added
the file to an in-memory FS via an API that cannot fail, use
`llvm_unreachable` on the error path. Didn't use an `assert` since it
seems cleaner semantically to check the error (and better,
hypothetically, for updating the API to use `Expected` instead of
`ErrorOr`).

I noticed this incidentally while auditing calls to `createFileID`.