[NFC] Rename TokenConsumptionHandle.missing to .tokenIsMissing

[WowbaggersLiquidLunch]

This clarifies what is missing at the use site. Also according the the API design guidelines:

Uses of Boolean methods and properties should read as assertions about the receiver when the use is nonmutating, e.g. x.isEmpty, line1.intersects(line2).

[WowbaggersLiquidLunch]

@swift-ci please test

[ahoppen]

Renaming missing -> isMissing makes sense to me. I’m not sure if we need to include token in there because at least to me it’s obvious that we’re talking about a token since only tokens can be missing in SwiftSyntax and this is a TokenConsumptionHandle. What did you think might have been missing? If it was ambiguous to you, I’m happy to change my mind.

[WowbaggersLiquidLunch]

this is a _Token_ConsumptionHandle

When I was first reading the source, all I saw was that all the handles were named just handles. It wasn't obvious to me that they were instances of _Token_ConsumptionHandle. I only figured out that it's the token that's missing when I jumped to definition and read the documentation. I wasn't able to deduce it without reading the documentation, because I didn't know what the handles were for to begin with. So it would be confusing and doesn't seem obvious to me (and especially to new contributors I guess) that the receiver/subject of isMissing is the token not the handle.

[ahoppen]

Ah, with that explanation the rename makes sense to me. Thanks.