[Demangle] Guard DEMANGLE_ABI and add missing annotation (#202920)

This updates the DEMANGLE_ABI annotation to only be defined if it is not
already defined. This is required to parse the Demangle headers with the
ids-check script.
In addition, this adds one missing DEMANGLE_ABI annotation.

This effort is tracked in #109483.

NOKEYCHECK=True
GitOrigin-RevId: 8210a58044d1e6d86473fdf810396d285c86ff36