[YAMLIO] Correctly diagnose empty alias/anchor

The `Range` of an alias/anchor token includes the leading `&` or `*`,
but it is skipped while parsing the name. The check for an empty name
fails to account for the skipped leading character and so the error is
never hit.

Fix the off-by-one and add a couple regression tests.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D91462