When we compare the APIs of entire SDKs, we can assume super classes
are always present in the API dump. However, this assumption is invalid
for framework-level API diff checking, where super classes can come from
other modules whose contents are not serialized.
rdar://33110442
In Jenkins bot, we've seen "exception std::length_error: vector" for
api-digester tests. Tentatively, this patch initializes a major vector with a
reasonably large size, which may avoid its wild capacity growth.
