[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
Fix the off-by-one and add a couple regression tests.
Reviewed By: dexonsmith
Differential Revision: https://reviews.llvm.org/D91462