It is common practice in C to declare anonymous tags that are
immediately given a typedef name, e.g.,
typedef enum { ... } MyType;
At present, one can only express API notes on the typedef. However, that
excludes the possibility of tag-specific notes like EnumExtensibility.
For these anonymous declarations, process API notes using the typedef
name as the tag name, so that one can add API notes to `MyType` via the
`Tags` section.
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