All the other semantic flags have plural names - ScopeFlags, SymbolFlags, NodeFlags. ReferenceFlag is the outlier.

Rename:

• ReferenceFlag to ReferenceFlags.
• Reference::flag field to flags.
• Reference::flag method to flags.
• Reference::flag_mut method to flags_mut.
• All vars called flag to flags.

Only reason I'm opening an issue for this rather than just submitting a PR is it'd cause merge conflicts with PR stack #4973, so will wait for that to get merged first.